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.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web-player/src/routes/(app)/mixes/[id]/+page.svelte

@@ -72,7 +72,7 @@
 		<div class="min-w-0 flex-1">
 			<p class="text-sm tracking-[0.14em] text-fg-3 uppercase">Mezcla</p>
 			<h1
-				class="mt-1.5 text-3xl font-semibold tracking-tight break-words text-fg sm:text-5xl md:text-7xl"
+				class="mt-1.5 font-display text-3xl font-semibold tracking-tight break-words text-fg sm:text-5xl md:text-7xl"
 			>
 				{mix.title}
 			</h1>