feat(surface): switch app shells when the viewport crosses the phone threshold

The mobile-vs-desktop surface is stamped once at boot, so a browser
window narrowed past the phone threshold kept the desktop shell (and
its legacy squeezed layout) until a manual reload. A viewport watcher
now reloads into the other shell once the resize settles — the same
mechanism the old Settings toggle used. Fixed shells (Capacitor,
desktop, VS Code) and ?surface= overrides never switch.

With the new mobile app reachable this way, the old/new mobile layout
preference is gone: phones always get the mobile app.

packages/ui/src/lib/i18n/messages/de.settings.ts

@@ -1845,9 +1845,6 @@ export const settingsDict = {
   'settings.voice.page.field.ttsInputModeRaw': 'Rohes Markdown',
   'settings.voice.page.field.ttsInputModeSummarized': 'zusammengefasst',
   'settings.openchamber.visual.section.colorMode': 'Farbmodus',
-  'settings.openchamber.visual.section.mobileLayout': 'Mobiles Layout',
-  'settings.openchamber.visual.option.mobileLayout.default': 'Alt',
-  'settings.openchamber.visual.option.mobileLayout.new': 'Neu',
   'settings.openchamber.visual.section.localization': 'Lokalisierung',
   'settings.openchamber.visual.section.spacingAndLayout': 'Abstand & Layout',
   'settings.openchamber.visual.section.navigation': 'Navigation',

packages/ui/src/lib/i18n/messages/en.settings.ts

@@ -1913,9 +1913,6 @@ export const settingsDict = {
   'settings.voice.page.field.ttsInputModeSummarized': 'summarized',
   'settings.openchamber.visual.section.colorMode': 'Color Mode',
   'settings.openchamber.visual.section.colorModeAndTheme': 'Color mode & Theme',
-  'settings.openchamber.visual.section.mobileLayout': 'Mobile Layout',
-  'settings.openchamber.visual.option.mobileLayout.default': 'Old',
-  'settings.openchamber.visual.option.mobileLayout.new': 'New',
   'settings.openchamber.visual.section.localization': 'Localization',
   'settings.openchamber.visual.section.spacingAndLayout': 'Spacing & Layout',
   'settings.openchamber.visual.section.densityAndType': 'Density & type',

packages/ui/src/lib/i18n/messages/es.settings.ts

@@ -1890,9 +1890,6 @@ export const settingsDict = {
   "settings.voice.page.field.ttsInputModeSummarized": "resumido",
   "settings.openchamber.visual.section.colorMode": "Modo de color",
   "settings.openchamber.visual.section.colorModeAndTheme": "Modo de color y tema",
-  "settings.openchamber.visual.section.mobileLayout": "Diseño móvil",
-  "settings.openchamber.visual.option.mobileLayout.default": "Anterior",
-  "settings.openchamber.visual.option.mobileLayout.new": "Nuevo",
   "settings.openchamber.visual.section.localization": "Localización",
   "settings.openchamber.visual.section.spacingAndLayout": "Espaciado y diseño",
   "settings.openchamber.visual.section.densityAndType": "Densidad y tipografía",

packages/ui/src/lib/i18n/messages/fr.settings.ts

@@ -2147,9 +2147,6 @@ export const settingsDict = {
   'settings.voice.page.field.ttsInputModeSanitized': 'Nettoyé',
   'settings.voice.page.field.ttsInputModeRaw': 'Markdown brut',
   'settings.voice.page.field.ttsInputModeSummarized': 'résumé',
-  'settings.openchamber.visual.section.mobileLayout': 'Mise en page mobile',
-  'settings.openchamber.visual.option.mobileLayout.default': 'Ancienne',
-  'settings.openchamber.visual.option.mobileLayout.new': 'Nouvelle',
   'settings.openchamber.visual.field.dockBadge': 'Badge du Dock',
   'settings.openchamber.visual.field.dockBadgeHint': 'Afficher sur l’icône du Dock de macOS le nombre de discussions avec une activité non vue.',
   'settings.openchamber.visual.actions.saveAndRestart': 'Enregistrer et redémarrer',

packages/ui/src/lib/i18n/messages/ja.settings.ts

@@ -1923,9 +1923,6 @@ export const settingsDict = {
   'settings.voice.page.field.ttsInputModeSummarized': '要約',
   'settings.openchamber.visual.section.colorMode': 'カラーモード',
   'settings.openchamber.visual.section.colorModeAndTheme': 'カラーモードとテーマ',
-  'settings.openchamber.visual.section.mobileLayout': 'モバイルレイアウト',
-  'settings.openchamber.visual.option.mobileLayout.default': '旧',
-  'settings.openchamber.visual.option.mobileLayout.new': '新',
   'settings.openchamber.visual.section.localization': 'ローカライゼーション',
   'settings.openchamber.visual.section.spacingAndLayout': '間隔とレイアウト',
   'settings.openchamber.visual.section.densityAndType': '密度と書体',

packages/ui/src/lib/i18n/messages/ko.settings.ts

@@ -1890,9 +1890,6 @@ export const settingsDict = {
   'settings.voice.page.field.ttsInputModeSummarized': '요약',
   'settings.openchamber.visual.section.colorMode': '색상 모드',
   'settings.openchamber.visual.section.colorModeAndTheme': '색상 모드 및 테마',
-  'settings.openchamber.visual.section.mobileLayout': '모바일 레이아웃',
-  'settings.openchamber.visual.option.mobileLayout.default': '이전',
-  'settings.openchamber.visual.option.mobileLayout.new': '새로움',
   'settings.openchamber.visual.section.localization': '지역화',
   'settings.openchamber.visual.section.spacingAndLayout': '간격 및 레이아웃',
   'settings.openchamber.visual.section.densityAndType': '밀도 및 서체',

packages/ui/src/lib/i18n/messages/pl.settings.ts

@@ -1196,9 +1196,6 @@ export const settingsDict = {
   'settings.openchamber.visual.section.chatFeatures': 'Funkcje',
   'settings.openchamber.visual.section.colorMode': 'Tryb kolorów',
   'settings.openchamber.visual.section.colorModeAndTheme': 'Tryb kolorów i motyw',
-  'settings.openchamber.visual.section.mobileLayout': 'Układ mobilny',
-  'settings.openchamber.visual.option.mobileLayout.default': 'Poprzedni',
-  'settings.openchamber.visual.option.mobileLayout.new': 'Nowy',
   'settings.openchamber.visual.section.diffLayout': 'Układ diffa',
   'settings.openchamber.visual.section.diffLayoutAria': 'Układ diffa',
   'settings.openchamber.visual.section.localization': 'Lokalizacja',

packages/ui/src/lib/i18n/messages/pt-BR.settings.ts

@@ -1890,9 +1890,6 @@ export const settingsDict = {
   "settings.voice.page.field.ttsInputModeSummarized": "resumido",
   "settings.openchamber.visual.section.colorMode": "Modo de cor",
   "settings.openchamber.visual.section.colorModeAndTheme": "Modo de cor e tema",
-  "settings.openchamber.visual.section.mobileLayout": "Layout móvel",
-  "settings.openchamber.visual.option.mobileLayout.default": "Anterior",
-  "settings.openchamber.visual.option.mobileLayout.new": "Novo",
   "settings.openchamber.visual.section.localization": "Localização",
   "settings.openchamber.visual.section.spacingAndLayout": "Espaçamento e layout",
   "settings.openchamber.visual.section.densityAndType": "Densidade e tipografia",

packages/ui/src/lib/i18n/messages/uk.settings.ts

@@ -1890,9 +1890,6 @@ export const settingsDict = {
   "settings.voice.page.field.ttsInputModeSummarized": "скорочений",
   "settings.openchamber.visual.section.colorMode": "Режим теми",
   "settings.openchamber.visual.section.colorModeAndTheme": "Режим кольору та тема",
-  "settings.openchamber.visual.section.mobileLayout": "Мобільний макет",
-  "settings.openchamber.visual.option.mobileLayout.default": "Попередній",
-  "settings.openchamber.visual.option.mobileLayout.new": "Новий",
   "settings.openchamber.visual.section.localization": "Локалізація",
   "settings.openchamber.visual.section.spacingAndLayout": "Відступи й компонування",
   "settings.openchamber.visual.section.densityAndType": "Щільність і шрифти",

packages/ui/src/lib/i18n/messages/zh-CN.settings.ts

@@ -1890,9 +1890,6 @@ export const settingsDict = {
   'settings.voice.page.field.ttsInputModeSummarized': '摘要',
   'settings.openchamber.visual.section.colorMode': '颜色模式',
   'settings.openchamber.visual.section.colorModeAndTheme': '颜色模式与主题',
-  'settings.openchamber.visual.section.mobileLayout': '移动端布局',
-  'settings.openchamber.visual.option.mobileLayout.default': '旧版',
-  'settings.openchamber.visual.option.mobileLayout.new': '新版',
   'settings.openchamber.visual.section.localization': '本地化',
   'settings.openchamber.visual.section.spacingAndLayout': '间距与布局',
   'settings.openchamber.visual.section.densityAndType': '密度与字体',

packages/ui/src/lib/i18n/messages/zh-TW.settings.ts

@@ -1801,7 +1801,6 @@ export const settingsDict = {
   'settings.openchamber.visual.section.spacingAndLayout': '間距與佈局',
   'settings.openchamber.visual.section.densityAndType': '密度與字型',
   'settings.openchamber.visual.section.appInstall': '應用程式安裝',
-  'settings.openchamber.visual.section.mobileLayout': '行動版版面',
   'settings.openchamber.visual.section.navigation': '導覽',
   'settings.openchamber.visual.section.chatRenderMode': '聊天渲染模式',
   'settings.openchamber.visual.section.chatRenderModeAria': '聊天渲染模式',
@@ -1855,8 +1854,6 @@ export const settingsDict = {
   'settings.openchamber.visual.field.mobileKeyboardModeAria': '行動裝置鍵盤行為',
   'settings.openchamber.visual.field.selectMobileKeyboardModePlaceholder': '選擇鍵盤行為',
   'settings.openchamber.visual.actions.resetMobileKeyboardModeAria': '重設行動裝置鍵盤行為',
-  'settings.openchamber.visual.option.mobileLayout.default': '舊版',
-  'settings.openchamber.visual.option.mobileLayout.new': '新版',
   'settings.openchamber.visual.field.interfaceFontSize': '介面字體大小',
   'settings.openchamber.visual.field.interfaceFont': '介面字體',
   'settings.openchamber.visual.field.selectInterfaceFontAria': '選擇介面字體',

packages/ui/src/lib/mobileLayoutPreference.ts

@@ -1,34 +0,0 @@
-export type MobileLayoutPreference = 'default' | 'new';
-
-const MOBILE_LAYOUT_PREFERENCE_KEY = 'openchamber-mobile-layout';
-
-const normalizeMobileLayoutPreference = (value: unknown): MobileLayoutPreference => {
-  // 'new' is the default; only an explicit 'default' (the legacy/"Old" layout)
-  // opts out of it.
-  return value === 'default' ? 'default' : 'new';
-};
-
-export const getStoredMobileLayoutPreference = (): MobileLayoutPreference => {
-  if (typeof window === 'undefined') {
-    return 'new';
-  }
-
-  try {
-    return normalizeMobileLayoutPreference(window.localStorage.getItem(MOBILE_LAYOUT_PREFERENCE_KEY));
-  } catch {
-    return 'new';
-  }
-};
-
-export const setStoredMobileLayoutPreference = (value: MobileLayoutPreference): boolean => {
-  if (typeof window === 'undefined') {
-    return false;
-  }
-
-  try {
-    window.localStorage.setItem(MOBILE_LAYOUT_PREFERENCE_KEY, value);
-    return true;
-  } catch {
-    return false;
-  }
-};

packages/ui/src/lib/runtimeSurface.ts

@@ -1,6 +1,5 @@
 import { isDesktopShell, isVSCodeRuntime } from '@/lib/desktop';
 import { isCapacitorApp } from '@/lib/platform';
-import { getStoredMobileLayoutPreference } from '@/lib/mobileLayoutPreference';
 
 export type HostedSurface = 'desktop' | 'mobile';
 
@@ -11,6 +10,7 @@ declare global {
 }
 
 const MOBILE_SURFACE_MAX_WIDTH = 768;
+const SURFACE_SWITCH_DEBOUNCE_MS = 800;
 
 const isTouchOrCoarsePointer = (): boolean => {
   if (typeof window === 'undefined') return false;
@@ -22,12 +22,29 @@ const isTouchOrCoarsePointer = (): boolean => {
   return coarsePointer || touchPoints > 0;
 };
 
+const hasSurfaceUrlOverride = (): boolean => {
+  if (typeof window === 'undefined') return false;
+  const override = new URLSearchParams(window.location.search).get('surface');
+  return override === 'mobile' || override === 'desktop';
+};
+
+/** Viewport half of the surface decision; re-evaluated on resize by the watcher. */
+const isPhoneViewport = (): boolean => {
+  if (typeof window === 'undefined') return false;
+  const width = Math.min(
+    window.innerWidth || Number.POSITIVE_INFINITY,
+    window.screen?.width || Number.POSITIVE_INFINITY,
+  );
+  return Number.isFinite(width)
+    && width <= MOBILE_SURFACE_MAX_WIDTH
+    && isTouchOrCoarsePointer();
+};
+
 /**
  * Single authority for the mobile-vs-desktop surface decision.
  *
  * Priority: explicit stamp (set once at boot) → URL override → Capacitor
- * shell (always the mobile surface) → desktop shells → phone heuristic
- * gated by the stored mobile layout preference.
+ * shell (always the mobile surface) → desktop shells → phone heuristic.
  */
 const detectHostedSurface = (): HostedSurface => {
   if (typeof window === 'undefined') return 'desktop';
@@ -45,14 +62,7 @@ const detectHostedSurface = (): HostedSurface => {
   if (isCapacitorApp()) return 'mobile';
   if (isDesktopShell() || isVSCodeRuntime()) return 'desktop';
 
-  const width = Math.min(
-    window.innerWidth || Number.POSITIVE_INFINITY,
-    window.screen?.width || Number.POSITIVE_INFINITY,
-  );
-  const likelyPhone = Number.isFinite(width)
-    && width <= MOBILE_SURFACE_MAX_WIDTH
-    && isTouchOrCoarsePointer();
-  return likelyPhone && getStoredMobileLayoutPreference() === 'new' ? 'mobile' : 'desktop';
+  return isPhoneViewport() ? 'mobile' : 'desktop';
 };
 
 /**
@@ -69,3 +79,39 @@ export const resolveHostedSurface = (): HostedSurface => {
 };
 
 export const isMobileSurfaceRuntime = (): boolean => detectHostedSurface() === 'mobile';
+
+/**
+ * The surface is stamped once at boot, so a browser window that crosses the
+ * phone threshold after load would otherwise keep the wrong app shell (the
+ * app trees, stores, and sync bootstrap differ, so an in-place switch is not
+ * safe). Watch for the viewport heuristic disagreeing with the stamp and
+ * reload — the same mechanism a surface change has always used — once the
+ * resize settles. Fixed shells never switch: Capacitor is always mobile,
+ * desktop/VS Code shells are always desktop, and an explicit ?surface=
+ * override wins over the heuristic.
+ */
+export const watchHostedSurfaceViewport = (): (() => void) => {
+  if (typeof window === 'undefined') return () => {};
+  if (isCapacitorApp() || isDesktopShell() || isVSCodeRuntime()) return () => {};
+  if (hasSurfaceUrlOverride()) return () => {};
+
+  let timeout: ReturnType<typeof setTimeout> | null = null;
+  const handleResize = () => {
+    if (timeout) clearTimeout(timeout);
+    timeout = setTimeout(() => {
+      timeout = null;
+      const stamped = window.__OPENCHAMBER_SURFACE__;
+      const desired: HostedSurface = isPhoneViewport() ? 'mobile' : 'desktop';
+      if (stamped && stamped !== desired) {
+        window.__OPENCHAMBER_SURFACE__ = undefined;
+        window.location.reload();
+      }
+    }, SURFACE_SWITCH_DEBOUNCE_MS);
+  };
+
+  window.addEventListener('resize', handleResize);
+  return () => {
+    if (timeout) clearTimeout(timeout);
+    window.removeEventListener('resize', handleResize);
+  };
+};