batch3 T5: frontend foundation — Pane types, panes API, user-events WS

- Mirror Pane/PaneState/UserStream types
- api.panes.* CRUD methods
- sessionEvents adds session_updated, session_loaded, open_file_in_browser
- useUserEvents hook: single app-level WS to /api/ws/user with reconnect
- useSidebar handles session_updated (in-place patch + re-sort) and
  session_loaded (active-project highlight gap fix); open_file_in_browser
  is a deliberate no-op here, consumed by Workspace later
- App.tsx mounts useUserEvents once

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/hooks/sessionEvents.ts

@@ -32,12 +32,34 @@ export interface SessionDeletedEvent {
   project_id: string;
 }
 
+export interface SessionUpdatedEvent {
+  type: 'session_updated';
+  session_id: string;
+  project_id: string;
+  name: string;
+  updated_at: string;
+}
+
+export interface SessionLoadedEvent {
+  type: 'session_loaded';
+  session_id: string;
+  project_id: string;
+}
+
+export interface OpenFileInBrowserEvent {
+  type: 'open_file_in_browser';
+  path: string; // project-relative
+}
+
 export type SessionEvent =
   | SessionRenamedEvent
   | ProjectCreatedEvent
   | ProjectDeletedEvent
   | SessionCreatedEvent
-  | SessionDeletedEvent;
+  | SessionDeletedEvent
+  | SessionUpdatedEvent
+  | SessionLoadedEvent
+  | OpenFileInBrowserEvent;
 type Listener = (event: SessionEvent) => void;
 
 const listeners = new Set<Listener>();