feat(settings): Session tabs toggle in General → Navigation

A new Session tabs group (web/desktop only) turns the header session
tabs off; disabled, the header renders the exact pre-tabs view — plain
session title with meta row and the always-visible session menu (the
same block VS Code uses). The Alt+W close-tab shortcut no-ops while
tabs are off. Registered in settings search with a matching anchor;
labels translated in all locales.

packages/ui/src/lib/settings/search.ts

@@ -161,6 +161,14 @@ const SETTINGS_SEARCH_ITEMS: readonly SettingsSearchItem[] = [
     titleKey: 'settings.openchamber.visual.field.fileEditorKeymap',
     keywords: ['editor', 'vim', 'keymap'],
   },
+  {
+    id: 'appearance.session-tabs',
+    page: 'general',
+    titleKey: 'settings.openchamber.visual.field.sessionTabsGroup',
+    descriptionKey: 'settings.openchamber.visual.field.sessionTabsInfo',
+    keywords: ['session', 'tabs', 'header', 'working set'],
+    isAvailable: (ctx) => !ctx.isMobile && !ctx.isVSCode,
+  },
   {
     id: 'appearance.terminal-quick-keys',
     page: 'general',