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/apps/officer-web/state/useInitialData.ts

@@ -1,13 +1,13 @@
 import { usePlans } from 'state/usePlans';
 import { useSettings } from 'state/useSettings';
-import { usePiModels } from 'state/useModels';
+import { useModels } from 'state/useModels';
 import { useAccessPolicy } from 'state/useAccessPolicy';
 import { useColorModeSync } from './useThemeSync';
 
 export const useInitialData = () => {
   const { plans } = usePlans();
   const { settings } = useSettings();
-  usePiModels();
+  useModels();
   useAccessPolicy();
   useColorModeSync();