chore: cleanup dead code and unused files

Deleted 7 unused frontend components (ChatBubble, PetZone, Live2DCat,
BackgroundScene, Clock, Toolbar, types/index.ts).
Deleted unused backend: whisper_stream.py, empty models/ and routers/ dirs.
Removed dead code: Message interface, messages/micError/sendText/addMessage,
conversation_text handler, 3 unused memory.py methods.
Fixed task persistence bug: added 'tasks' to DEFAULT_PREFERENCES.
Stale comment in Live2DStage updated.

backend/main.py

@@ -383,29 +383,6 @@ async def gemini_voice_ws(websocket: WebSocket):
                             await gemini_ws.send(json.dumps(gemini_msg))
                         continue
 
-                    if msg_type == "conversation_text":
-                        text = msg.get("text", "").strip()
-                        if not text:
-                            continue
-                        logger.info(f"[{session_id}] User (text): {text}")
-                        if gemini_ws and gemini_ws.state.name == "OPEN":
-                            user_part = {"text": text}
-                            if memory_suffix:
-                                user_part = {"text": f"[Context: {memory_suffix}]\n{text}"}
-                            gemini_msg = {
-                                "clientContent": {
-                                    "turns": [{"role": "user", "parts": [user_part]}],
-                                    "turnComplete": True,
-                                }
-                            }
-                            await gemini_ws.send(json.dumps(gemini_msg))
-                            await websocket.send_json({
-                                "type": "transcript",
-                                "role": "user",
-                                "text": text,
-                            })
-                        continue
-
                     if msg_type == "ping":
                         await websocket.send_json({"type": "pong"})