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/src-tauri/capabilities/default.json

@@ -2,6 +2,16 @@
   "$schema": "../gen/schemas/desktop-schema.json",
   "identifier": "default",
   "description": "Default capabilities for OpenChamber desktop runtime",
+  "remote": {
+    "urls": [
+      "http://127.0.0.1:*/*",
+      "http://localhost:*/*",
+      "http://*",
+      "http://*/*",
+      "https://*",
+      "https://*/*"
+    ]
+  },
   "windows": ["main"],
   "permissions": [
     "core:default",
@@ -20,24 +30,12 @@
     "dialog:allow-message",
     "dialog:allow-ask",
     "dialog:allow-confirm",
-    "fs:allow-read-text-file",
-    "fs:allow-read-file",
-    "fs:allow-write-text-file",
-    "fs:allow-write-file",
-    "fs:allow-read-dir",
-    "fs:allow-exists",
-    "fs:allow-create",
-    "fs:allow-mkdir",
-    "fs:allow-remove",
-    "fs:scope-app-index",
-    "fs:scope-home",
     "notification:default",
     "notification:allow-is-permission-granted",
     "notification:allow-request-permission",
     "notification:allow-notify",
     "updater:default",
     "updater:allow-check",
-    "updater:allow-download-and-install",
-    "process:allow-restart"
+    "updater:allow-download-and-install"
   ]
 }