feat(fs-lyrics): fade bottom edge of plain lyrics scroll viewport

Synced lyrics always fill the screen due to auto-scroll, so the
existing fsa-fade-bottom overlay naturally fades content below
the active line. Plain (unsynced) lyrics start from the top and
may not fill the viewport, leaving no content for that overlay
to fade.

Add mask-image to .fsa-lyrics-container when rendering plain
lyrics (--plain modifier class). The mask operates on the visible
scroll viewport rather than the content, so the bottom fade is
always present regardless of scroll position.

src/components/FullscreenPlayer.tsx

@@ -143,9 +143,11 @@ const FsLyricsApple = memo(function FsLyricsApple({ currentTrack }: { currentTra
 
   if (!currentTrack || loading) return null;
 
+  const isPlain = !hasSynced && !!plainLyrics;
+
   return (
     <div
-      className="fsa-lyrics-container"
+      className={`fsa-lyrics-container${isPlain ? ' fsa-lyrics-container--plain' : ''}`}
       ref={setContainerRef}
       onWheel={handleUserScroll}
       onTouchMove={handleUserScroll}

src/styles/components.css

@@ -3621,6 +3621,12 @@
   pointer-events: none;
 }
 
+/* Fade the bottom of the scroll viewport when showing plain lyrics */
+.fsa-lyrics-container--plain {
+  -webkit-mask-image: linear-gradient(to bottom, black 60%, transparent 100%);
+  mask-image: linear-gradient(to bottom, black 60%, transparent 100%);
+}
+
 .fsa-plain-line {
   font-size: clamp(1.1rem, 2.5vh, 2.2rem);
   font-weight: 500;