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/sections/usage/UsageSidebar.tsx

@@ -43,18 +43,8 @@ export const UsageSidebar: React.FC<UsageSidebarProps> = ({ onItemSelect }) => {
   const loadUsageSettings = useQuotaStore((state) => state.loadSettings);
   const { isMobile } = useDeviceInfo();
 
-  const [isDesktopRuntime, setIsDesktopRuntime] = React.useState<boolean>(() => {
-    if (typeof window === 'undefined') return false;
-    return typeof window.opencodeDesktop !== 'undefined';
-  });
-
   const isVSCode = React.useMemo(() => isVSCodeRuntime(), []);
 
-  React.useEffect(() => {
-    if (typeof window === 'undefined') return;
-    setIsDesktopRuntime(typeof window.opencodeDesktop !== 'undefined');
-  }, []);
-
   React.useEffect(() => {
     void loadUsageSettings();
   }, [loadUsageSettings]);
@@ -89,14 +79,7 @@ export const UsageSidebar: React.FC<UsageSidebarProps> = ({ onItemSelect }) => {
     void persistUsageSettings({ usageDisplayMode: value });
   }, [persistUsageSettings, setUsageDisplayMode]);
 
-
-
-
-  const bgClass = isDesktopRuntime
-    ? 'bg-transparent'
-    : isVSCode
-      ? 'bg-background'
-      : 'bg-sidebar';
+  const bgClass = isVSCode ? 'bg-background' : 'bg-sidebar';
 
   return (
     <div className={cn('flex h-full flex-col', bgClass)}>