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/theme/theme.css

@@ -36,6 +36,7 @@
 	--mf-logo-glow: 0 0 20px oklch(0.74 0.145 145 / 0.3);
 	--mf-cover-grad: linear-gradient(150deg, oklch(0.5 0.13 145), oklch(0.16 0.035 145));
 	--mf-liked-grad: linear-gradient(150deg, oklch(0.4 0.09 330), oklch(0.18 0.04 10));
+	--mf-tile-shade: rgba(9, 9, 12, 0.95);
 
 	--mf-accent: oklch(0.74 0.145 145);
 	--mf-accent-title: oklch(0.87 0.07 145);