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/routes/+page.svelte

@@ -155,7 +155,7 @@
 	></div>
 	<div class="animate-enter relative flex flex-col gap-2 sm:gap-2.5">
 		<p class="text-sm tracking-[0.14em] text-fg-2 uppercase sm:text-base">{data.greeting}</p>
-		<h1 class="text-3xl leading-[1.05] font-semibold text-fg sm:text-6xl">
+		<h1 class="font-display text-3xl leading-[1.05] font-semibold tracking-tight text-fg sm:text-6xl">
 			Tu música. Sin límites.
 		</h1>
 	</div>