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/routes/layout.css

@@ -32,8 +32,6 @@
 	--color-danger-fg: oklch(88.5% 0.062 18.334);
 	--color-btn: var(--mf-btn-bg);
 	--color-btn-hover: var(--mf-btn-bg-hover);
-	--color-btn-glass: var(--mf-btn-secondary-bg);
-	--color-btn-glass-hover: var(--mf-btn-secondary-bg-hover);
 
 	--radius-tag: 0.25rem;
 	--radius-thumb: 0.5rem;