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/chat/ChatContainer.tsx

@@ -30,7 +30,7 @@ export const ChatContainer: React.FC = () => {
         isSyncing,
         messageStreamStates,
         trimToViewportWindow,
-        sessionActivityPhase,
+        sessionStatus,
         newSessionDraft,
     } = useSessionStore();
 
@@ -161,8 +161,8 @@ export const ChatContainer: React.FC = () => {
             try {
                 await loadMessages(currentSessionId);
             } finally {
-                const currentPhase = sessionActivityPhase?.get(currentSessionId) ?? 'idle';
-                const isActivePhase = currentPhase === 'busy' || currentPhase === 'cooldown';
+                const statusType = sessionStatus?.get(currentSessionId)?.type ?? 'idle';
+                const isActivePhase = statusType === 'busy' || statusType === 'retry';
                 // When pinned and active, scroll is already maintained automatically
                 const shouldSkipScroll = isActivePhase && isPinned;
 
@@ -179,7 +179,7 @@ export const ChatContainer: React.FC = () => {
         };
 
         void load();
-    }, [currentSessionId, isPinned, loadMessages, messages, scrollToBottom, sessionActivityPhase]);
+    }, [currentSessionId, isPinned, loadMessages, messages, scrollToBottom, sessionStatus]);
 
     if (!currentSessionId && !draftOpen) {
         return (
@@ -266,14 +266,7 @@ export const ChatContainer: React.FC = () => {
                     <ScrollShadow
                         className="absolute inset-0 overflow-y-auto overflow-x-hidden z-0 chat-scroll overlay-scrollbar-target"
                         ref={scrollRef}
-                        style={{
-                            contain: 'strict',
-                            ['--scroll-shadow-size' as string]: '48px',
-                            // GPU acceleration hints for smoother scrolling
-                            transform: 'translateZ(0)',
-                            willChange: 'scroll-position',
-                            backfaceVisibility: 'hidden',
-                        }}
+                        observeMutations={false}
                         data-scroll-shadow="true"
                         data-scrollbar="chat"
                     >