feat(web): scroll-row arrows + stable desktop media tile

Home rows get hover left/right arrows that only show for the scrollable
direction (enhanceScrollRow + initScrollRows, called from startApp);
visibility recomputed on scroll/resize/content-load.

Fix the OS media tile flicker when skipping on desktop PWA: mutate the
existing MediaMetadata in place instead of assigning a new one each track
(the replacement made Windows tear down and rebuild the tile). Action
handlers are now set once.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/player/player.js

@@ -1019,23 +1019,38 @@ export function setVolume(v) {
 
 // ─── MEDIA SESSION ────────────────────────────────────────────────────────
 
+let _msHandlersSet = false;
 export function updateMediaSession(s) {
   // When native is active, Media3 owns the system notification — don't touch it.
   if (HAS_BRIDGE) return;
-  if(!navigator.mediaSession) return;
+  const ms = navigator.mediaSession;
+  if(!ms) return;
   const artwork = s.coverArt ? [
     { src: _cachedCoverUrl(s.coverArt, 256),  sizes:'256x256',   type:'image/jpeg' },
     { src: _cachedCoverUrl(s.coverArt, 512),  sizes:'512x512',   type:'image/jpeg' },
     { src: _cachedCoverUrl(s.coverArt, 1024), sizes:'1024x1024', type:'image/jpeg' },
   ] : [];
-  navigator.mediaSession.metadata = new MediaMetadata({ title:s.title, artist:s.artist||'', album:s.album||'', artwork });
-  navigator.mediaSession.setActionHandler('play',          togglePlay);
-  navigator.mediaSession.setActionHandler('pause',         togglePlay);
-  navigator.mediaSession.setActionHandler('previoustrack', prevSong);
-  navigator.mediaSession.setActionHandler('nexttrack',     nextSong);
-  navigator.mediaSession.setActionHandler('seekbackward', d => { S.audio.currentTime = Math.max(0, S.audio.currentTime-(d.seekOffset||10)); });
-  navigator.mediaSession.setActionHandler('seekforward',  d => { S.audio.currentTime = Math.min(S.audio.duration||0, S.audio.currentTime+(d.seekOffset||10)); });
-  navigator.mediaSession.setActionHandler('seekto',       d => { if(d.seekTime!=null) S.audio.currentTime = d.seekTime; });
+  // Mutate the existing metadata in place rather than assigning a new MediaMetadata —
+  // replacing the object makes Windows tear down & rebuild the media tile (visible
+  // flicker when skipping). Spotify-style: keep one object, just update its fields.
+  if (ms.metadata) {
+    ms.metadata.title   = s.title;
+    ms.metadata.artist  = s.artist || '';
+    ms.metadata.album   = s.album || '';
+    ms.metadata.artwork = artwork;
+  } else {
+    ms.metadata = new MediaMetadata({ title:s.title, artist:s.artist||'', album:s.album||'', artwork });
+  }
+  // Action handlers are stable — set them once, not on every track change.
+  if (_msHandlersSet) return;
+  _msHandlersSet = true;
+  ms.setActionHandler('play',          togglePlay);
+  ms.setActionHandler('pause',         togglePlay);
+  ms.setActionHandler('previoustrack', prevSong);
+  ms.setActionHandler('nexttrack',     nextSong);
+  ms.setActionHandler('seekbackward', d => { S.audio.currentTime = Math.max(0, S.audio.currentTime-(d.seekOffset||10)); });
+  ms.setActionHandler('seekforward',  d => { S.audio.currentTime = Math.min(S.audio.duration||0, S.audio.currentTime+(d.seekOffset||10)); });
+  ms.setActionHandler('seekto',       d => { if(d.seekTime!=null) S.audio.currentTime = d.seekTime; });
 }
 
 export function syncMediaSessionPosition() {