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/api/server-settings/pi-mono.ts

@@ -1,8 +1,8 @@
 import { join } from 'node:path';
 import { createRouter } from '../../create-router';
 import { PI_CONFIG_DIR } from '../../data-path';
-import { invalidateModelCache } from '../pi/list-models';
-import { logger } from '../pi/logger';
+import { invalidateModelCache } from '../chat/list-models';
+import { logger } from '../chat/logger';
 import { readConfigValue, writeConfigValue } from 'officerdb';
 
 export const piMonoRouter = createRouter();