Make the secondary button adapt to shifting backgrounds

secondary used a flat, static tint that only worked over a plain
surface, so it looked out of place on the hero's hue-shifting gradient.
Add a backdrop blur (no border, per feedback) so it always reads
against whatever's actually behind it. Gave the button its own
background tokens instead of aliasing --mf-surface-2, and dropped the
now-unused --color-btn-glass tokens left over from the removed glass
variant.

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 backdrop-blur-md text-fg-2 hover:bg-btn-hover hover:text-fg',
 		accent: 'bg-accent-btn text-accent-soft hover:bg-accent-btn-hover',
 		danger: 'bg-danger text-on-art hover:brightness-110'
 	};