Polish typography, button states, and login screen

Applies the display typeface (already used for small section labels)
consistently to every major heading — page titles, hero titles on
album/playlist/mix pages, the home hero, the sidebar/mobile wordmark, and
empty-state titles — instead of only the smaller ones, fixing an
inconsistent visual hierarchy. Also fixes a couple of real bugs found along
the way: Input had a focus border color with no border width to show it,
Button/IconButton had no focus-visible ring at all, and SectionHeading's
letter-spacing only applied when it had an actions slot.

Button's secondary/subtle variants drop their harsh border and flat grey
hover for a softer accent-tinted wash. The accent (and the per-track
extracted accent) is desaturated for a calmer, more pastel feel without
touching lightness, so on-accent text contrast is unaffected.

The login screen gets the same treatment plus a subtle ambient glow (reusing
the home page's existing accent-gradient technique) and a fluid,
viewport-scaled headline instead of a few fixed breakpoints, so it fills the
available space on wide screens instead of staying pinned to one size.

web-player/src/lib/theme/palette.ts

@@ -100,7 +100,7 @@ export async function extractAccent(url: string): Promise<Accent | null> {
 	if (best < 0) return null;
 
 	const hue = ((Math.atan2(hueSin[best], hueCos[best]) * 180) / Math.PI + 360) % 360;
-	const chroma = Math.min(0.19, Math.max(0.1, chromaSum[best] / weight[best]));
+	const chroma = Math.min(0.11, Math.max(0.045, chromaSum[best] / weight[best]));
 
 	const h = hue.toFixed(1);
 	const c = chroma.toFixed(3);