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/components/Sidebar.svelte

@@ -82,7 +82,7 @@
 	class="sticky top-0 hidden h-screen shrink-0 flex-col gap-7 border-r border-line px-4.5 py-6.5 md:flex"
 	style="width:var(--mf-sidebar-w)"
 >
-	<a href="/" class="px-3 text-xl font-medium tracking-wide">Musify</a>
+	<a href="/" class="px-3 font-display text-xl font-semibold tracking-tight text-fg">Musify</a>
 
 	<div class="relative flex flex-col gap-7" bind:this={navBox}>
 		<div