refactor(web-player): centralise styles and drop dead code

Styles: adds the scrim and on-art tokens so the black overlays and the
white marks drawn over artwork stop being raw tailwind colours, and
routes every component through the semantic layer instead of reaching
into the --mf-* palette. Renames --mf-accent-60, which was actually an
80% mix, to --mf-accent-soft, and removes the unused --mf-radius and
the --color-neutral-950 override.

Dead code: the root +layout.server.ts already redirects everything
except /login, so the anonymous branches in the home and explore loads
and the needsAuth empty state could never run. Deletes the unused
accentSoft helper, and stops exporting helpers nobody imports.

fmtTime and fmtDate move out of theme/color.ts into $lib/format.ts;
they are not colour concerns.

The accent animation now cancels its pending frame on teardown.

web-player/src/lib/components/ui/Button.svelte

@@ -38,7 +38,7 @@
 		primary: 'bg-accent-soft font-semibold text-on-accent active:scale-[0.97]',
 		secondary: 'border border-line text-fg hover:bg-hover',
 		subtle: 'bg-surface-2 font-semibold text-fg hover:bg-surface-hover',
-		danger: 'bg-danger font-semibold text-white hover:brightness-110'
+		danger: 'bg-danger font-semibold text-on-accent hover:brightness-110'
 	};
 
 	const classes = $derived(

web-player/src/lib/components/ui/Modal.svelte

@@ -68,7 +68,7 @@
 
 {#if open}
 	<div
-		class="animate-fade fixed inset-0 z-[60] grid place-items-center bg-black/70 p-4 backdrop-blur-sm"
+		class="animate-fade fixed inset-0 z-[60] grid place-items-center bg-scrim/70 p-4 backdrop-blur-sm"
 		role="presentation"
 		onclick={onBackdropClick}
 	>

web-player/src/lib/components/ui/NowPlaying.svelte

@@ -13,10 +13,10 @@
 	];
 </script>
 
-<div class="absolute inset-0 flex items-end justify-center gap-[2.5px] bg-black/45 pb-2">
+<div class="absolute inset-0 flex items-end justify-center gap-[2.5px] bg-scrim/45 pb-2">
 	{#each bars as bar, i (i)}
 		<span
-			class="w-[3px] origin-bottom rounded-full bg-white"
+			class="w-[3px] origin-bottom rounded-full bg-on-art"
 			style="height:14px; animation:equalize {bar.dur}s ease-in-out {bar.delay}s infinite; animation-play-state:{paused
 				? 'paused'
 				: 'running'}"