Fix light-mode regression and finish design-system cleanup

A code-review pass on the previous commit found a real bug: the explore
genre tiles used var(--mf-ink) as a decorative gradient endpoint, but that
token is meant for text-on-accent and flips to white in light mode,
turning the tiles solid white. Replaces it with a new, deliberately
theme-invariant --mf-tile-shade token (same category as --mf-cover-grad).

Also: raises CollectionHeader's meta-line contrast in light mode
(text-muted, ~3:1, to text-fg-2), makes its actions prop optional so
pages without conditional actions don't render an empty wrapper, moves
AccountMenu.svelte into $lib/components/ui/ per the repo's own
componentization rule, adds the aria-expanded TopBar's trigger was
missing relative to MobileHeader's, and migrates the profile page
(u/[id]/+page.svelte) onto CollectionHeader — a sixth duplicate of the
same header pattern that the previous commit touched but missed.

web-player/src/lib/components/TopBar.svelte

@@ -7,7 +7,7 @@
 	import X from '@lucide/svelte/icons/x';
 	import ChevronLeft from '@lucide/svelte/icons/chevron-left';
 	import ChevronRight from '@lucide/svelte/icons/chevron-right';
-	import AccountMenu from './AccountMenu.svelte';
+	import AccountMenu from './ui/AccountMenu.svelte';
 
 	interface Props {
 		user: SessionUser | null;
@@ -107,11 +107,12 @@
 				{accountUrl}
 				panelClass="absolute top-full right-0 z-20 mt-2 w-52 overflow-hidden"
 			>
-				{#snippet trigger({ toggle })}
+				{#snippet trigger({ toggle, open })}
 					<button
 						type="button"
 						onclick={toggle}
 						aria-label="Tu cuenta"
+						aria-expanded={open}
 						class="block h-8.25 w-8.25 shrink-0 overflow-hidden rounded-full border border-line-strong transition active:scale-95"
 					>
 						{#if user.picture}