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/sidecar/claude/user-instance.ts

@@ -172,7 +172,7 @@ async function handleCommand(cmd: SidecarCommand, reply: ReplyFn) {
     case 'claude:spawn-streaming': {
       reply({ type: 'claude:spawned', id: cmd.id, sessionKey: cmd.params.sessionKey });
 
-      const onEvent = (event: import('../../api/pi/types').PiEvent) => {
+      const onEvent = (event: import('../../api/chat/types').ChatEvent) => {
         connection.send({ type: 'claude:event', sessionKey: cmd.params.sessionKey, event });
       };