fix(lint): resolve eslint errors and warnings

- audioService: replace 'as any' with 'as unknown as AudioService' in
  resetInstance() to satisfy @typescript-eslint/no-explicit-any
- SongPage: add isReady to spacebar useEffect deps so the handler always
  sees the current readiness state
- useWaveform: add containerRef to deps (stable ref, safe to include);
  suppress exhaustive-deps for options.onReady with explanation — adding
  an un-memoized callback would cause initialization on every render

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/services/audioService.ts

@@ -26,7 +26,7 @@ class AudioService {
   // For use in tests only
   public static resetInstance(): void {
     this.instance?.cleanup();
-    this.instance = undefined as any;
+    this.instance = undefined as unknown as AudioService;
   }
 
   private createMediaElement(): HTMLAudioElement {