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/nginx.conf

@@ -4,6 +4,9 @@ server {
     root /usr/share/nginx/html;
     index index.html;
 
+    # Max upload size
+    client_max_body_size 15M;
+
     # Gzip compression
     gzip on;
     gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;
@@ -12,10 +15,18 @@ server {
     location /api {
         proxy_pass http://backend:8000;
         proxy_http_version 1.1;
-        proxy_set_header Host $host;
+        proxy_set_header Host $http_host;
         proxy_set_header X-Real-IP $remote_addr;
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
         proxy_set_header X-Forwarded-Proto $scheme;
+        proxy_set_header Range $http_range;
+        proxy_set_header If-Range $http_if_range;
+        proxy_redirect off;
+
+        # Disable buffering for streaming
+        proxy_buffering off;
+        proxy_cache off;
+        proxy_request_buffering off;
     }
 
     # WebSocket proxy