Brighten secondary button text for dark-mode legibility

secondary used text-fg-2 (--mf-text-2), which in dark mode is a fairly
dim mid-gray (#8a8a8e) that read poorly against the dark bg-btn fill,
even though the same class looks great in light mode where --mf-text-2
is a dark, high-contrast gray. Use text-fg instead, which is tuned
bright in dark mode and dark in light mode, so both modes get good
contrast from the same class. Drop the now-redundant hover:text-fg
since resting state already uses it.

web-player/src/lib/components/ui/primitives/Button.svelte

@@ -36,7 +36,7 @@
 
 	const variants = {
 		primary: 'bg-cta-strong text-ink hover:brightness-95',
-		secondary: 'bg-btn text-fg-2 hover:bg-btn-hover hover:text-fg',
+		secondary: 'bg-btn text-fg hover:bg-btn-hover',
 		accent: 'bg-accent-tint text-accent-soft hover:bg-accent-btn-hover',
 		danger: 'bg-danger text-on-art hover:brightness-110'
 	};