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/useUserEvents.ts

@@ -0,0 +1,58 @@
+import { useEffect } from 'react';
+import { sessionEvents } from './sessionEvents';
+
+const RECONNECT_INITIAL_MS = 1000;
+const RECONNECT_MAX_MS = 30000;
+
+export function useUserEvents(): void {
+  useEffect(() => {
+    let ws: WebSocket | null = null;
+    let reconnectTimer: ReturnType<typeof setTimeout> | null = null;
+    let reconnectDelay = RECONNECT_INITIAL_MS;
+    let unmounted = false;
+
+    const connect = () => {
+      if (unmounted) return;
+      const url = new URL('/api/ws/user', window.location.href);
+      url.protocol = url.protocol === 'https:' ? 'wss:' : 'ws:';
+      ws = new WebSocket(url.toString());
+
+      ws.onopen = () => {
+        reconnectDelay = RECONNECT_INITIAL_MS;
+      };
+
+      ws.onmessage = (ev) => {
+        try {
+          const frame = JSON.parse(ev.data);
+          // The server emits frames whose `type` matches SessionEvent union members
+          // (project_created, project_deleted, session_created, session_deleted, session_updated).
+          // Pass through onto the bus.
+          sessionEvents.emit(frame);
+        } catch (err) {
+          console.warn('useUserEvents: failed to parse frame', err);
+        }
+      };
+
+      ws.onclose = () => {
+        if (unmounted) return;
+        reconnectTimer = setTimeout(() => {
+          reconnectDelay = Math.min(reconnectDelay * 2, RECONNECT_MAX_MS);
+          connect();
+        }, reconnectDelay);
+      };
+
+      ws.onerror = () => {
+        // close handler will trigger reconnect
+        try { ws?.close(); } catch {}
+      };
+    };
+
+    connect();
+
+    return () => {
+      unmounted = true;
+      if (reconnectTimer) clearTimeout(reconnectTimer);
+      if (ws) try { ws.close(); } catch {}
+    };
+  }, []);
+}