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>

package.json

@@ -8,7 +8,8 @@
     "preview": "vite preview",
     "tauri": "tauri",
     "tauri:dev": "GDK_BACKEND=x11 WEBKIT_DISABLE_COMPOSITING_MODE=1 tauri dev",
-    "tauri:build": "tauri build"
+    "tauri:build": "tauri build",
+    "test": "vitest run"
   },
   "dependencies": {
     "@tanstack/react-virtual": "^3.13.23",
@@ -39,6 +40,7 @@
     "@types/react-dom": "^18.3.1",
     "@vitejs/plugin-react": "^4.3.2",
     "typescript": "^5.5.3",
-    "vite": "^6.0.3"
+    "vite": "^6.0.3",
+    "vitest": "^4.1.3"
   }
 }