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/main.js

@@ -3,6 +3,7 @@ import { toast } from './utils.js';
 import { api, fetchRomajiIndex, loadRomajiOverrides } from './api.js';
 import { injectPWA } from './pwa.js';
 import { loadSavedTheme } from './themes.js';
+import { initScrollRows } from './ui/components.js';
 import {
   playQueue, playCurrent, togglePlay, prevSong, nextSong, onEnded,
   toggleShuffle, toggleRepeat, toggleQueue, setVolume, seekTo,
@@ -221,6 +222,7 @@ window.addEventListener('load', () => {
   injectPWA();
   initProgressScrub();
   initKeyboardShortcuts();
+  initScrollRows();
 
   // Click player-art to open Now Playing
   document.getElementById('player-art')?.addEventListener('click', openNowPlaying);