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/api/client.ts

@@ -7,6 +7,9 @@ import type {
   SidebarResponse,
   ListDirResult,
   ViewFileResult,
+  Pane,
+  PaneCreateRequest,
+  PaneUpdateRequest,
 } from './types';
 
 export class ApiError extends Error {
@@ -113,4 +116,23 @@ export const api = {
   sidebar: {
     get: () => request<SidebarResponse>('/api/sidebar'),
   },
+
+  panes: {
+    getForSession: (sessionId: string) =>
+      request<{ panes: Pane[] }>(`/api/sessions/${sessionId}/panes`),
+    create: (sessionId: string, body: PaneCreateRequest) =>
+      request<Pane>(`/api/sessions/${sessionId}/panes`, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify(body),
+      }),
+    update: (id: string, body: PaneUpdateRequest) =>
+      request<Pane>(`/api/panes/${id}`, {
+        method: 'PATCH',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify(body),
+      }),
+    remove: (id: string) =>
+      request<void>(`/api/panes/${id}`, { method: 'DELETE' }),
+  },
 };

apps/web/src/api/types.ts

@@ -64,6 +64,7 @@ export interface SidebarSession {
   name: string;
   model: string;
   updated_at: string;
+  project_id: string;
 }
 
 export interface SidebarProject {
@@ -96,6 +97,36 @@ export interface ViewFileResult {
   bytes_returned: number;
 }
 
+export type PaneKind = 'chat' | 'file_browser';
+
+export interface FileBrowserPaneState {
+  open_file?: string | null;
+  filter?: string;
+  expanded_dirs?: string[];
+}
+export type ChatPaneState = Record<string, never>;
+export type PaneState = ChatPaneState | FileBrowserPaneState;
+
+interface PaneBase {
+  id: string;
+  session_id: string;
+  position: number;
+  created_at: string;
+}
+export type Pane = PaneBase & (
+  | { kind: 'chat'; state: ChatPaneState }
+  | { kind: 'file_browser'; state: FileBrowserPaneState }
+);
+
+export interface PaneCreateRequest {
+  kind: PaneKind;
+  position?: number;
+}
+export interface PaneUpdateRequest {
+  state?: PaneState;
+  position?: number;
+}
+
 export type WsFrame =
   | { type: 'snapshot'; messages: Message[] }
   | { type: 'message_started'; message_id: string; role: MessageRole }