Unify web-player design tokens and componentize collection headers

Migrates remaining one-off colors (bg-white/[N%], border-white/N, hardcoded
shadows/gradients) to the existing --mf-* tokens so light mode reacts
consistently everywhere, including a fix to --color-hover which was
hardcoded instead of pointing at --mf-surface-2. Extracts CollectionHeader
(album/playlist/mix/liked headers) and AccountMenu (TopBar/MobileHeader)
to remove five and two duplicated implementations respectively, and
normalizes inconsistent empty-state copy.

web-player/src/lib/components/ui/ImageDropzone.svelte

@@ -57,6 +57,6 @@
 			class="h-full w-full object-cover"
 		/>
 	{:else}
-		<Icon class="h-9 w-9 {gradient ? 'text-white/70' : 'text-muted'}" strokeWidth={1.3} />
+		<Icon class="h-9 w-9 {gradient ? 'text-on-art/70' : 'text-muted'}" strokeWidth={1.3} />
 	{/if}
 </label>