Tie secondary button color to the page background instead of a flat tint

secondary was fully bg-transparent (invisible at rest) or a flat neutral
overlay (clashed next to accent-tinted buttons and looked different
depending on what surface it sat on). Use bg-elevated instead, which is
already derived from the same hue as --mf-bg, so the button is always a
faint, minimal step above whatever page background it's on, and shifts
with it automatically.

Drop the now-unused --color-btn/--mf-btn-bg tokens.

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 backdrop-blur-md text-fg-2 hover:bg-btn-hover hover:text-fg',
+		secondary: 'bg-elevated 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'
 	};