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/routes/layout.css

@@ -30,7 +30,6 @@
 	--color-on-art-2: var(--mf-on-art-2);
 	--color-danger: oklch(63.7% 0.15 25.331);
 	--color-danger-fg: oklch(88.5% 0.062 18.334);
-	--color-btn: var(--mf-btn-bg);
 	--color-btn-hover: var(--mf-btn-bg-hover);
 
 	--radius-tag: 0.25rem;