feat(fs-player): adaptive dynamic accent color from album cover

Extracts the most vibrant pixel from an 8×8 downscaled album cover via
the Canvas API and applies it as --dynamic-fs-accent on the .fs-player
root element.  All accent-colored FS player elements use
var(--dynamic-fs-accent, var(--accent)) as fallback so the theme accent
is restored automatically when the FS player closes.

Elements updated:
- .fs-track-title (color + text-shadow)
- .fs-btn.active, .fs-btn-heart.active/:hover
- .fs-btn-play and :hover (background + box-shadow)
- .fs-seekbar-played (background + box-shadow)
- .fs-mesh-blob-a/-b (radial-gradient via color-mix at 14%/8% opacity)
- .fs-art-wrap box-shadow glow (color-mix at 70% opacity)
- MicVocal lyrics-toggle inline style

Color safety: WCAG 4.5:1 contrast against near-black FS background
enforced by progressively lightening in HSL space (ensureContrast).

Pure math functions unit-tested with vitest (28 tests, 0 deps beyond
vitest itself).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

vitest.config.ts

@@ -0,0 +1,8 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    environment: 'node',
+    include: ['src/**/*.test.ts'],
+  },
+});