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/channels/send-claude-code.ts

@@ -1,5 +1,5 @@
-import { logger } from '@@/api/pi/logger';
-import type { MessageCost, PiEvent } from '@@/api/pi/types';
+import { logger } from '@@/api/chat/logger';
+import type { MessageCost, ChatEvent } from '@@/api/chat/types';
 import * as sidecar from '@@/sidecar-registry';
 
 type ClaudeCodeParams = {
@@ -40,7 +40,7 @@ type ClaudeCodeStreamingParams = {
   model?: string;
   role?: string;
   resumeSessionId?: string;
-  onEvent: (event: PiEvent) => void;
+  onEvent: (event: ChatEvent) => void;
 };
 
 type ClaudeCodeStreamingHandle = {