Improve mini-player and add periodic sync

- Redesign mini-player: progress bar on top, centered controls
- Add vertical volume slider with popup on hover
- Add volume percentage display
- Add custom speaker SVG icons
- Add periodic sync every 10 seconds for playback synchronization
- Broadcast user_joined when connecting via WebSocket
- Disable nginx proxy buffering for streaming
- Allow extra env variables in pydantic settings

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/routers/websocket.py

@@ -46,6 +46,13 @@ async def room_websocket(websocket: WebSocket, room_id: UUID):
 
     await manager.connect(websocket, room_id, user.id)
 
+    # Notify others that user joined
+    await manager.broadcast_to_room(
+        room_id,
+        {"type": "user_joined", "user": {"id": str(user.id), "username": user.username}},
+        exclude_user=user.id
+    )
+
     try:
         while True:
             data = await websocket.receive_text()