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>

frontend/public/speaker-disabled-svgrepo-com.svg

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?><!-- Uploaded to: SVG Repo, www.svgrepo.com, Generator: SVG Repo Mixer Tools -->
+<svg width="800px" height="800px" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M5 8.14307C3.4148 8.66137 3 9.49393 3 10.5V13.5C3 14.6046 3.5 15.5 5.5 16C7.5 16.5 9 21 12 21C12.6098 21 13.0337 19.3265 13.2717 17M3 3L21 21M9 4.60756C9.84604 3.71548 10.8038 3 12 3C12.7739 3 13.2484 5.69533 13.4233 9" stroke="#000000" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>