Make the hero/spotlight surface tint more transparent

--mf-hero-bg/--mf-spotlight-bg's overlay tint was at 16% alpha, which
after the pastel lightness/chroma boost read as a fairly solid block
of color rather than a soft wash. Halve it to 8% so more of the plain
page background shows through underneath.

Keep the secondary button's color-mix ratio (in layout.css) in sync at
8% too, since it's built to replicate exactly what the surface behind
it looks like.

web-player/src/routes/layout.css

@@ -33,7 +33,7 @@
 	--color-btn: color-mix(
 		in srgb,
 		var(--mf-elevated) 45%,
-		color-mix(in srgb, var(--mf-hero-tint) 16%, var(--mf-bg))
+		color-mix(in srgb, var(--mf-hero-tint) 8%, var(--mf-bg))
 	);
 	--color-btn-hover: var(--mf-btn-bg-hover);