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/lib/debug.ts

@@ -216,13 +216,7 @@ export const debugUtils = {
     const runtimeApis = typeof window !== 'undefined'
       ? (window as any).__OPENCHAMBER_RUNTIME_APIS__
       : null;
-    const desktopServer = typeof window !== 'undefined'
-      ? (window as any).__OPENCHAMBER_DESKTOP_SERVER__
-      : null;
-    const isDesktopRuntime = Boolean(
-      runtimeApis?.runtime?.isDesktop ||
-      (typeof window !== 'undefined' && (window as any).opencodeDesktop)
-    );
+    const isTauriShell = typeof window !== 'undefined' && Boolean((window as any).__TAURI__);
 
     const safeJson = async (resp: Response) => {
       try {
@@ -302,10 +296,10 @@ export const debugUtils = {
     const report = {
       runtime: {
         platform: runtimeApis?.runtime?.platform ?? null,
-        isDesktop: isDesktopRuntime,
+        isDesktop: isTauriShell,
         isVSCode: Boolean(runtimeApis?.runtime?.isVSCode),
         hasRuntimeApis: Boolean(runtimeApis),
-        desktopServerOrigin: desktopServer?.origin ?? null,
+        desktopServerOrigin: null,
       },
       location: typeof window !== 'undefined'
         ? {