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/desktop/package.json

@@ -12,29 +12,15 @@
     "tauri": "tauri",
     "tauri:dev": "tauri dev --features devtools",
     "tauri:build": "tauri build",
-    "dev": "vite dev --host 127.0.0.1 --port 1421",
-    "build": "vite build",
-    "preview": "vite preview --host 127.0.0.1 --port 5051",
-    "type-check": "tsc --noEmit",
-    "lint": "eslint \"./src/**/*.{ts,tsx}\" --config ../../eslint.config.js"
-  },
-  "dependencies": {
-    "@tauri-apps/plugin-notification": "^2.3.3",
-    "@tauri-apps/plugin-process": "^2",
-    "@tauri-apps/plugin-updater": "^2",
-    "@openchamber/ui": "workspace:*",
-    "react": "^19.1.1",
-    "react-dom": "^19.1.1"
+    "build:sidecar": "node ./scripts/build-sidecar.mjs",
+    "build": "bun -e \"process.exit(0)\"",
+    "type-check": "bun -e \"process.exit(0)\"",
+    "lint": "bun -e \"process.exit(0)\""
   },
+  "dependencies": {},
   "devDependencies": {
     "@tauri-apps/cli": "^2",
-    "@tauri-apps/api": "^2.9.1",
-    "@tauri-apps/plugin-dialog": "^2.4.2",
     "@types/node": "^24.3.1",
-    "@types/react": "^19.1.10",
-    "@types/react-dom": "^19.1.7",
-    "@vitejs/plugin-react": "^5.0.0",
-    "typescript": "~5.8.3",
-    "vite": "^7.1.2"
+    "typescript": "~5.8.3"
   }
 }