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/routes/(app)/explore/+page.svelte

@@ -477,7 +477,7 @@
 					href={buildHref(genre.query)}
 					class="animate-enter group relative block h-28 overflow-hidden rounded-2xl p-4.25 transition-[filter] hover:brightness-125"
 					style="animation-delay:{Math.min(i, 10) *
-						40}ms; background:linear-gradient(140deg, oklch(0.24 0.07 {genre.hue}), var(--mf-ink))"
+						40}ms; background:linear-gradient(140deg, oklch(0.24 0.07 {genre.hue}), var(--mf-tile-shade))"
 				>
 					<div class="font-display text-base font-semibold tracking-[-0.02em] text-fg">
 						{genre.label}