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/lib/components/ui/IconButton.svelte

@@ -29,7 +29,7 @@
 	{onclick}
 	aria-label={label}
 	title={label}
-	class="grid place-items-center rounded-control text-muted transition hover:bg-hover hover:text-fg {sizes[
+	class="grid place-items-center rounded-control text-muted transition hover:bg-hover hover:text-fg focus-visible:bg-hover focus-visible:text-fg focus-visible:ring-2 focus-visible:ring-accent focus-visible:outline-none {sizes[
 		size
 	]} {revealOnHover
 		? 'sm:opacity-0 sm:group-hover:opacity-100 sm:focus-visible:opacity-100'