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/config.py

@@ -25,6 +25,7 @@ class Settings(BaseSettings):
 
     class Config:
         env_file = ".env"
+        extra = "ignore"
 
 
 @lru_cache()