fix(audio): prevent WKWebView audio session teardown after backgrounding (#41) (#486)

Keep a silent looping <audio> element mounted at the app root so macOS
never tears down the CoreAudio session. Without this, backgrounding the
app long enough leaves WaveSurfer's AudioContext in a state where play()
resolves and timeupdate fires, but no audio reaches the output — and not
even cmd+R (full JS reload) restores it, only a full app relaunch.

Uses a zero-PCM WAV blob at full volume rather than a muted element,
since WebKit can optimize muted media away and defeat the purpose.

Co-authored-by: Claude Opus 4.7 (1M context) <[email protected]>

app/src/components/AppFrame/AppFrame.tsx

@@ -1,5 +1,6 @@
 import { useRouterState } from '@tanstack/react-router';
 import { TitleBarDragRegion } from '@/components/TitleBarDragRegion';
+import { AudioKeepAlive } from '@/components/AudioPlayer/AudioKeepAlive';
 import { AudioPlayer } from '@/components/AudioPlayer/AudioPlayer';
 import { StoryTrackEditor } from '@/components/StoriesTab/StoryTrackEditor';
 import { TOP_SAFE_AREA_PADDING } from '@/lib/constants/ui';
@@ -26,6 +27,7 @@ export function AppFrame({ children }: AppFrameProps) {
       className={cn('h-screen bg-background flex flex-col overflow-hidden', TOP_SAFE_AREA_PADDING)}
     >
       <TitleBarDragRegion />
+      <AudioKeepAlive />
       {children}
       {showTrackEditor ? (
         <StoryTrackEditor storyId={story.id} items={story.items} />