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/auth/SessionAuthGate.tsx

@@ -2,9 +2,10 @@ import React from 'react';
 import { RiLockLine, RiLockUnlockLine, RiLoader4Line } from '@remixicon/react';
 import { Button } from '@/components/ui/button';
 import { Input } from '@/components/ui/input';
-import { isDesktopRuntime, isVSCodeRuntime } from '@/lib/desktop';
+import { isDesktopShell, isVSCodeRuntime } from '@/lib/desktop';
 import { syncDesktopSettings, initializeAppearancePreferences } from '@/lib/persistence';
 import { applyPersistedDirectoryPreferences } from '@/lib/directoryPersistence';
+import { DesktopHostSwitcherInline } from '@/components/desktop/DesktopHostSwitcher';
 
 const STATUS_CHECK_ENDPOINT = '/auth/session';
 
@@ -119,9 +120,9 @@ const clearTokenFromUrl = () => {
 };
 
 export const SessionAuthGate: React.FC<SessionAuthGateProps> = ({ children }) => {
-  const desktopRuntime = React.useMemo(() => isDesktopRuntime(), []);
   const vscodeRuntime = React.useMemo(() => isVSCodeRuntime(), []);
-  const skipAuth = desktopRuntime || vscodeRuntime;
+  const skipAuth = vscodeRuntime;
+  const showHostSwitcher = React.useMemo(() => isDesktopShell() && !vscodeRuntime, [vscodeRuntime]);
   const [state, setState] = React.useState<GateState>(() => (skipAuth ? 'authenticated' : 'pending'));
   const [password, setPassword] = React.useState('');
   const [isSubmitting, setIsSubmitting] = React.useState(false);
@@ -349,6 +350,15 @@ export const SessionAuthGate: React.FC<SessionAuthGateProps> = ({ children }) =>
               </p>
             )}
           </form>
+
+          {showHostSwitcher && (
+            <div className="w-full">
+              <DesktopHostSwitcherInline />
+              <p className="mt-1 text-center typography-micro text-muted-foreground">
+                Use Local if remote is unreachable.
+              </p>
+            </div>
+          )}
         </div>
       </AuthShell>
     );