feat: add Chinese UI, Responses models, and mirrored Windows setup

Localize the web console and persist custom model and capability settings.
Use OpenAI Responses exclusively and move runtime limits into environment configuration.
Bootstrap uv and the virtual environment on Windows, support optional China mirrors,
and package source distributions without environments, build output, or credentials.

Update regression tests and repository quality checks for the new behavior.

playground/backend_manager/app/services/session_manager.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 import asyncio
-from datetime import datetime, timedelta, timezone
+from datetime import datetime, timedelta, timezone, UTC
 import logging
 import uuid
 from app.config import settings
@@ -49,7 +49,7 @@ class SessionManagerService:
         while True:
             try:
                 await asyncio.sleep(settings.REAPER_CHECK_INTERVAL_SECONDS)
-                now = datetime.now(timezone.utc)
+                now = datetime.now(UTC)
                 active_sessions = await bigquery_service.list_all_active_sessions()
 
                 for s in active_sessions:
@@ -78,7 +78,7 @@ class SessionManagerService:
     async def create_session(self, user_id: str, request: CreateSessionRequest) -> SessionResponse:
         """Provision a new Cuttlefish emulator + Artemis container and link them via ADB."""
         session_id = str(uuid.uuid4())
-        now = datetime.now(timezone.utc)
+        now = datetime.now(UTC)
         expires_at = now + timedelta(minutes=request.ttl_minutes)
 
         logger.info(
@@ -164,7 +164,7 @@ class SessionManagerService:
         if not record or record.user_id != user_id or record.status == SessionStatus.TERMINATED:
             return None
 
-        now = datetime.now(timezone.utc)
+        now = datetime.now(UTC)
         record.last_heartbeat_at = now
         record.expires_at = now + timedelta(minutes=settings.SESSION_TTL_MINUTES)
         await bigquery_service.save_session(record)