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/services/s3.py

@@ -110,5 +110,11 @@ def stream_file_chunks(s3_key: str, start: int = 0, end: int = None, chunk_size:
         Range=range_header
     )
 
-    for chunk in response["Body"].iter_chunks(chunk_size=chunk_size):
+    # Use raw stream read instead of iter_chunks for true streaming
+    body = response["Body"]
+    while True:
+        chunk = body.read(chunk_size)
+        if not chunk:
+            break
         yield chunk
+    body.close()