feat(frontend): rework audio player and shared UI components

Expand the player store, split player controls (transport, seek, volume, track info), add Cover/PlaylistArt/NowPlaying UI primitives and a cover image proxy route.

frontend/src/lib/components/ui/Cover.svelte

@@ -0,0 +1,35 @@
+<script lang="ts">
+	import type { Snippet } from 'svelte';
+	import { gradientForHue } from '$lib/theme/color';
+
+	interface Props {
+		trackId: string | number;
+		hue: number;
+		size?: 'small' | 'medium' | 'large';
+		alt?: string;
+		class?: string;
+		children?: Snippet;
+	}
+
+	let { trackId, hue, size = 'medium', alt = '', class: klass = '', children }: Props = $props();
+
+	let failed = $state(false);
+	let loaded = $state(false);
+</script>
+
+<div
+	class="relative overflow-hidden {klass}"
+	style={loaded && !failed ? undefined : `background:${gradientForHue(hue)}`}
+>
+	{#if !failed}
+		<img
+			src="/api/tracks/{trackId}/cover?size={size}"
+			{alt}
+			loading="lazy"
+			onload={() => (loaded = true)}
+			onerror={() => (failed = true)}
+			class="h-full w-full object-cover"
+		/>
+	{/if}
+	{@render children?.()}
+</div>