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-1-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="M16 8.99998C16.5 9.49998 17 10.5 17 12C17 13.5 16.5 14.5 16 15M3 10.5V13.5C3 14.6046 3.5 15.5 5.5 16C7.5 16.5 9 21 12 21C14 21 14 3 12 3C9 3 7.5 7.5 5.5 8C3.5 8.5 3 9.39543 3 10.5Z" stroke="#000000" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>