perf(fs-player): cut CPU under WebKitGTK software compositing

WebKitGTK on Linux runs with WEBKIT_DISABLE_COMPOSITING_MODE=1, so every
GPU-only effect is software-rasterised per frame. The fullscreen player
piled up large blur shadows, per-line scale transforms, oversized
radial-gradients with color-mix() and a 60 fps rAF spring on top — CPU
saturated and FPS dropped to ~10.

Linux-only via existing html.no-compositing class:
- Apple-style lyrics: drop transform: scale and 48 px / 32 px text-shadow
- Rail-style lyrics: drop 28 px text-shadow + scaleX
- Track title: 40 px glow → 6 px drop-shadow
- Album art wrap + play button: drop accent-coloured outer glows
- Mesh blobs: flatten 130 % radial-gradient + color-mix to solid #06060e
  and remove the 400 ms background transition
- Portrait wrap: drop translateZ(0) GPU-layer hint
- FS art: instant cover swap (no opacity crossfade)
- SpringScroller: replace rAF spring with native scrollTo({behavior:'smooth'})
- FsSeekbar: cache last applied values, skip identical DOM writes

Dynamic accent on title / seekbar / play button is preserved.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/utils/springScroll.ts

@@ -40,6 +40,15 @@ export class SpringScroller {
 
   scrollTo(targetY: number) {
     this.target = Math.max(0, targetY);
+    // Software-rendered Linux: replace our 60 fps rAF spring with the
+    // browser's native smooth scroll. The browser handles easing internally
+    // (one animation, not 60 JS callbacks per second) and we still get a
+    // smooth visual instead of a hard snap.
+    if (document.documentElement.classList.contains('no-compositing')) {
+      this.stop();
+      this.container.scrollTo({ top: this.target, behavior: 'smooth' });
+      return;
+    }
     if (this.rafId === null) this.tick();
   }