Fix multiple issues: format API, Gradio availability, storage, UI responsiveness

- Format endpoint now calls ACE-Step /format_input REST API directly instead of
  spawning Python, fixing ENOENT errors on Windows (#44, #27, #34)
- isGradioAvailable() tries /gradio_api/info, /info, / in sequence to handle
  Gradio 4.x/5.x/6.x version differences, fixing generation fallback (#53, #20)
- Storage getUrl/getPublicUrl normalize /audio/ prefix to prevent double-prefix
  URLs when reference tracks are used for cover generation (#10)
- Gradio args: fix normalization_db default from 0.0 to -1.0 (Gradio default)
- Volume popover: add 400ms delay before hiding to prevent accidental dismissal (#51)
- Polling: skip setSongs state update when nothing changed to reduce re-renders (#51)
- FFmpeg: add jsdelivr CDN fallback when unpkg fails (#30)
- Model switching: add switchModelIfNeeded() via /v1/init REST API (#45)

components/Player.tsx

@@ -69,6 +69,7 @@ export const Player: React.FC<PlayerProps> = ({
     const progressBarRef = useRef<HTMLDivElement>(null);
     const fullscreenProgressRef = useRef<HTMLDivElement>(null);
     const [isHoveringVolume, setIsHoveringVolume] = useState(false);
+    const volumeHideTimer = useRef<ReturnType<typeof setTimeout> | null>(null);
     const [showDropdown, setShowDropdown] = useState(false);
     const [isFullscreen, setIsFullscreen] = useState(false);
     const [shareModalOpen, setShareModalOpen] = useState(false);
@@ -755,8 +756,13 @@ export const Player: React.FC<PlayerProps> = ({
                     {/* Volume Control with Vertical Slider */}
                     <div
                         className="relative group hidden md:block"
-                        onMouseEnter={() => setIsHoveringVolume(true)}
-                        onMouseLeave={() => setIsHoveringVolume(false)}
+                        onMouseEnter={() => {
+                            if (volumeHideTimer.current) clearTimeout(volumeHideTimer.current);
+                            setIsHoveringVolume(true);
+                        }}
+                        onMouseLeave={() => {
+                            volumeHideTimer.current = setTimeout(() => setIsHoveringVolume(false), 400);
+                        }}
                     >
                         <button
                             onClick={() => onVolumeChange(volume === 0 ? 0.8 : 0)}