chat: rename the pi chat transport + model list to chat (Stage 4b/1)

Pure rename, no behavior change. Moves the misnamed "pi" chat harness into the
chat namespace:

- api/pi/{websocket,session-manager,types,logger,list-models} → api/chat/
- merge api/pi/rest.ts into api/chat/chat.ts (/pi/models → /chat/models,
  /pi/stt → /chat/stt); drop the piRestRouter mount
- PiEvent → ChatEvent, piWebsocket → chatWebsocket, listPiModels → listChatModels
- WS route /api/pi/chat/ws → /api/chat/ws, provider tag 'pi' → 'chat'
- frontend: useChat/useAudioRecording URLs, usePiModels→useModels /
  useVisiblePiModels→useVisibleModels / useEnabledPiModels→useEnabledModels,
  'PI_MODELS' query key → 'CHAT_MODELS', attachments provider 'pi-mono' → 'chat'

The /pi-mono provider/harness settings router is renamed separately (next commit).
Note: the WS route change requires the mobile app to point at /api/chat/ws.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/servers/hono.ts

@@ -19,7 +19,6 @@ import { settingsRouter } from './api/settings/settings';
 import { dashboardsRouter } from './api/dashboards';
 import { taskLogsRouter } from './api/task-logs/task-logs';
 import { router as fileBrowserRouter } from './api/file-browser/router';
-import { piRestRouter } from './api/pi/rest';
 import { devServerRouter, devServerProxyRouter } from './api/dev-server/router';
 import { dockRouter } from './api/dock/dock';
 import { integrationsRouter, googleCallbackHandler } from './api/integrations/integrations';
@@ -107,7 +106,6 @@ protectedRouter.route('/pipeline-jobs', pipelineJobsRouter);
 protectedRouter.route('/jobs', pipelineJobsRouter); // unified jobs API (script + pipeline); /pipeline-jobs kept for the existing UI
 desktopRouter.use(superAdminMiddleware);
 protectedRouter.route('/desktop', desktopRouter);
-protectedRouter.route('/', piRestRouter);
 
 honoServer.route('/api', protectedRouter);