Match the accent button's background to the active-row highlight

Button's accent variant used bg-accent-btn (15% alpha) while the
active-track highlight in ListRow/TrackList uses bg-accent-tint (8%
alpha) with the same text-accent-soft title color, so a button like
"Reproducir" sat at a visibly different tint than the row it plays.
Use bg-accent-tint for accent's resting background too, so both read
as the same accent color; hover still steps up to bg-accent-btn-hover.

web-player/src/lib/components/ui/primitives/Button.svelte

@@ -37,7 +37,7 @@
 	const variants = {
 		primary: 'bg-cta-strong text-ink hover:brightness-95',
 		secondary: 'bg-btn text-fg-2 hover:bg-btn-hover hover:text-fg',
-		accent: 'bg-accent-btn text-accent-soft hover:bg-accent-btn-hover',
+		accent: 'bg-accent-tint text-accent-soft hover:bg-accent-btn-hover',
 		danger: 'bg-danger text-on-art hover:brightness-110'
 	};