refactor(desktop): make Tauri thin shell running web sidecar (#273)

## What / Why
This PR finishes the desktop refactor: the Tauri app is now a thin shell that launches the web server as a sidecar and loads the UI from `http://127.0.0.1:<port>`. All real backend logic lives in `packages/web/server/index.js`; desktop Rust keeps only native integrations (menu/dialog/notifications/updater/deep-link + window chrome).
This unblocks:
- consistent behavior across web/desktop/vscode (single backend)
- simpler desktop maintenance (no duplicated Rust backend)
- host switching between Local + remote instances in desktop
- reliable cold-start behavior on slow machines (VSCode + desktop)
## Key changes
- Desktop sidecar runtime
  - build pipeline to bundle web dist + `openchamber-server` sidecar (`packages/desktop/scripts/build-sidecar.mjs`)
  - robust local port selection (prefer saved/default, fallback to random; persisted in `~/.config/openchamber/settings.json`)
  - improved PATH handling so the sidecar can locate `opencode` CLI (incl `~/.opencode/bin`, overrides, common bins)
  - disable native right-click context menu in production builds (dev keeps it)
- Desktop instance switcher (Tauri-only)
  - header button + modal to add/edit/delete remote hosts, set default, probe status/ping, switch back to Local escape hatch
  - auth gate includes host switcher so you can recover when a remote host is broken/auth-required
  - host list stored desktop-locally (not tied to the currently selected remote server)
- Notifications
  - decision logic moved server-side; desktop notifications emitted via sidecar stdout and shown natively by Tauri
  - prevent double-notifications on desktop Local origin (UI ignores SSE notification when native path is active)
  - restore macOS notification sound
- Updates
  - Tauri updater used only when viewing Local instance in desktop shell (avoid “remote web update” triggering desktop restart)
- Settings persistence & UX polish
  - persist model favorites/recents via `/api/config/settings` (works for web + desktop; not origin-dependent)
  - persist per-project sidebar collapse state in `projects[].sidebarCollapsed` via `/api/config/settings` (with debounce on toggles)
  - macOS header sizing/traffic-lights offsets fixed (marketing macOS major injected from desktop; MultiRun header aligned)
  - VSCode cold-start: keep retrying provider/agent loads after connection to avoid empty UI on slow machines
  - misc lint/type fixes + bun.lock sync
- Desktop bootstrap / resiliency
  - show onboarding screen when OpenCode CLI is missing (desktop Local origin), with retry hook to restart OpenCode after install
## Testing notes
- Desktop (macOS): switch Local <-> remote, set default host, verify auth gate recovery, native notifications (with sound), updater gated to Local
- Web: favorites/recents + per-project collapsed state persist across reload/restart
- VSCode: slow startup no longer results in missing providers/agents/models

packages/ui/src/components/multirun/MultiRunLauncher.tsx

@@ -17,6 +17,7 @@ import type { CreateMultiRunParams, MultiRunModelSelection } from '@/types/multi
 import { ModelMultiSelect, generateInstanceId, type ModelSelectionWithId } from './ModelMultiSelect';
 import { BranchSelector, useBranchOptions } from './BranchSelector';
 import { AgentSelector } from './AgentSelector';
+import { isDesktopShell } from '@/lib/desktop';
 
 /** Max file size in bytes (10MB) */
 const MAX_FILE_SIZE = 10 * 1024 * 1024;
@@ -95,7 +96,7 @@ export const MultiRunLauncher: React.FC<MultiRunLauncherProps> = ({
     if (typeof window === 'undefined') {
       return false;
     }
-    return typeof (window as typeof window & { opencodeDesktop?: unknown }).opencodeDesktop !== 'undefined';
+    return isDesktopShell();
   });
 
   const isMacPlatform = React.useMemo(() => {
@@ -109,8 +110,33 @@ export const MultiRunLauncher: React.FC<MultiRunLauncherProps> = ({
     if (typeof window === 'undefined') {
       return;
     }
-    const detected = typeof (window as typeof window & { opencodeDesktop?: unknown }).opencodeDesktop !== 'undefined';
-    setIsDesktopApp(detected);
+    setIsDesktopApp(isDesktopShell());
+  }, []);
+
+  const macosMajorVersion = React.useMemo(() => {
+    if (typeof window === 'undefined') {
+      return null;
+    }
+
+    const injected = (window as unknown as { __OPENCHAMBER_MACOS_MAJOR__?: unknown }).__OPENCHAMBER_MACOS_MAJOR__;
+    if (typeof injected === 'number' && Number.isFinite(injected) && injected > 0) {
+      return injected;
+    }
+
+    // Fallback: WebKit reports "Mac OS X 10_15_7" format where 10 is legacy prefix
+    if (typeof navigator === 'undefined') {
+      return null;
+    }
+    const match = (navigator.userAgent || '').match(/Mac OS X (\d+)[._](\d+)/);
+    if (!match) {
+      return null;
+    }
+    const first = Number.parseInt(match[1], 10);
+    const second = Number.parseInt(match[2], 10);
+    if (Number.isNaN(first)) {
+      return null;
+    }
+    return first === 10 ? second : first;
   }, []);
 
   const desktopHeaderPaddingClass = React.useMemo(() => {
@@ -121,6 +147,19 @@ export const MultiRunLauncher: React.FC<MultiRunLauncherProps> = ({
     return 'pl-3';
   }, [isDesktopApp, isMacPlatform]);
 
+  const macosHeaderSizeClass = React.useMemo(() => {
+    if (!isDesktopApp || !isMacPlatform || macosMajorVersion === null) {
+      return '';
+    }
+    if (macosMajorVersion >= 26) {
+      return 'h-12';
+    }
+    if (macosMajorVersion <= 15) {
+      return 'h-14';
+    }
+    return '';
+  }, [isDesktopApp, isMacPlatform, macosMajorVersion]);
+
   const handleDragStart = React.useCallback(async (e: React.MouseEvent) => {
     if ((e.target as HTMLElement).closest('button, a, input, select, textarea')) {
       return;
@@ -321,7 +360,8 @@ export const MultiRunLauncher: React.FC<MultiRunLauncherProps> = ({
         onMouseDown={handleDragStart}
         className={cn(
           'relative flex h-12 items-center justify-center border-b app-region-drag select-none',
-          desktopHeaderPaddingClass
+          desktopHeaderPaddingClass,
+          macosHeaderSizeClass,
         )}
         style={{ borderColor: 'var(--interactive-border)' }}
       >