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/layout/Header.tsx

@@ -34,6 +34,8 @@ import { UsageProgressBar } from '@/components/sections/usage/UsageProgressBar';
 import { updateDesktopSettings } from '@/lib/persistence';
 import type { UsageWindow } from '@/types';
 import type { GitHubAuthStatus } from '@/lib/api/types';
+import { DesktopHostSwitcherButton } from '@/components/desktop/DesktopHostSwitcher';
+import { isDesktopShell } from '@/lib/desktop';
 
 const formatTime = (timestamp: number | null) => {
   if (!timestamp) return '-';
@@ -124,7 +126,7 @@ export const Header: React.FC = () => {
     if (typeof window === 'undefined') {
       return false;
     }
-    return typeof (window as typeof window & { opencodeDesktop?: unknown }).opencodeDesktop !== 'undefined';
+    return isDesktopShell();
   });
 
   const isMacPlatform = React.useMemo(() => {
@@ -138,11 +140,12 @@ export const Header: React.FC = () => {
     if (typeof window === 'undefined') {
       return null;
     }
-    // Use Tauri-provided version if available (accurate), otherwise fall back to UA parsing
-    const desktopApi = (window as typeof window & { opencodeDesktop?: { macosMajorVersion?: number | null } }).opencodeDesktop;
-    if (desktopApi?.macosMajorVersion != null) {
-      return desktopApi.macosMajorVersion;
+
+    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;
@@ -163,8 +166,7 @@ export const Header: React.FC = () => {
     if (typeof window === 'undefined') {
       return;
     }
-    const detected = typeof (window as typeof window & { opencodeDesktop?: unknown }).opencodeDesktop !== 'undefined';
-    setIsDesktopApp(detected);
+    setIsDesktopApp(isDesktopShell());
   }, []);
 
   const currentModel = getCurrentModel();
@@ -625,6 +627,9 @@ export const Header: React.FC = () => {
       <div className="flex-1" />
 
       <div className="flex items-center gap-1 pr-3">
+        {isDesktopApp && (
+          <DesktopHostSwitcherButton headerIconButtonClass={headerIconButtonClass} />
+        )}
         <Tooltip delayDuration={500}>
           <TooltipTrigger asChild>
             <button
@@ -770,7 +775,6 @@ export const Header: React.FC = () => {
             ))}
           </DropdownMenuContent>
         </DropdownMenu>
-
         <McpDropdown headerIconButtonClass={headerIconButtonClass} />
 
         <Tooltip delayDuration={500}>