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)

server/src/services/storage/local.ts

@@ -22,14 +22,13 @@ export class LocalStorageProvider implements StorageProvider {
   }
 
   async getUrl(key: string, _expiresIn?: number): Promise<string> {
-    return `/audio/${key}`;
+    const cleanKey = key.startsWith('/audio/') ? key.slice('/audio/'.length) : key;
+    return `/audio/${cleanKey.replace(/^\/+/, '')}`;
   }
 
   getPublicUrl(key: string): string {
-    if (key.startsWith('/audio/')) {
-      return key;
-    }
-    return `/audio/${key}`;
+    const cleanKey = key.startsWith('/audio/') ? key.slice('/audio/'.length) : key;
+    return `/audio/${cleanKey.replace(/^\/+/, '')}`;
   }
 
   async delete(key: string): Promise<void> {