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>

styles.css

@@ -168,6 +168,24 @@ nav{flex:1;overflow-y:auto;padding:4px 10px}
 .scroll-row::-webkit-scrollbar{display:none}
 .scroll-row .card{flex-shrink:0;width:145px}
 
+/* Hover arrows for horizontal rows (desktop) — only the scrollable direction shows */
+.scroll-row-wrap{position:relative}
+.scroll-arrow{
+  position:absolute;top:72px;transform:translateY(-50%);
+  width:38px;height:38px;border-radius:50%;
+  display:flex;align-items:center;justify-content:center;
+  background:rgba(15,15,24,.82);color:#fff;border:1px solid rgba(255,255,255,.14);
+  cursor:pointer;z-index:6;opacity:0;pointer-events:none;
+  transition:opacity .16s ease,background .16s ease;
+  -webkit-backdrop-filter:blur(8px);backdrop-filter:blur(8px);
+  box-shadow:0 4px 14px rgba(0,0,0,.35);
+}
+.scroll-arrow svg{width:18px;height:18px}
+.scroll-arrow.sa-left{left:-6px}
+.scroll-arrow.sa-right{right:-6px}
+.scroll-arrow:hover{background:rgba(30,30,46,.95)}
+.scroll-row-wrap:hover .scroll-arrow.can-scroll{opacity:1;pointer-events:auto}
+
 /* ── CARDS ── */
 .card{background:var(--surface);border:1px solid var(--border);border-radius:var(--radius);overflow:hidden;cursor:pointer;transition:transform .2s,box-shadow .2s,border-color .2s}
 .card:hover{transform:translateY(-3px);box-shadow:0 12px 32px rgba(0,0,0,.4);border-color:var(--border2)}