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/theme.css

@@ -15,7 +15,7 @@
 	--mf-border-strong: rgba(255, 255, 255, 0.12);
 	--mf-track: rgba(255, 255, 255, 0.12);
 
-	--mf-accent: oklch(60% 0.17 145);
+	--mf-accent: oklch(60% 0.1 145);
 	--mf-accent-soft: color-mix(in oklch, var(--mf-accent) 80%, transparent);
 
 	--mf-scrim: #000000;