feat(immersive): use full artwork background like Cider

Replace blur approach with full resolution artwork:
- Artwork scaled 115% to hide edges
- Strong vignette overlay fading to black at edges
- Gradient overlays for text readability (header, controls, lyrics)
- Slight desaturation for better contrast
- Simpler and more visually impactful

src/lib/components/immersive/ImmersivePlayer.svelte

@@ -214,7 +214,7 @@
     tabindex="-1"
   >
     <!-- Background -->
-    <ImmersiveBackground {artwork} mode="blur" />
+    <ImmersiveBackground {artwork} />
 
     <!-- Header with tabs -->
     <ImmersiveHeader