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/NowPlaying.svelte

@@ -0,0 +1,25 @@
+<script lang="ts">
+	interface Props {
+		paused?: boolean;
+	}
+
+	let { paused = false }: Props = $props();
+
+	const bars = [
+		{ delay: 0, dur: 0.9 },
+		{ delay: 0.25, dur: 0.7 },
+		{ delay: 0.1, dur: 1.05 },
+		{ delay: 0.4, dur: 0.8 }
+	];
+</script>
+
+<div class="absolute inset-0 flex items-end justify-center gap-[2.5px] bg-black/45 pb-2">
+	{#each bars as bar, i (i)}
+		<span
+			class="w-[3px] origin-bottom rounded-full bg-white"
+			style="height:14px; animation:equalize {bar.dur}s ease-in-out {bar.delay}s infinite; animation-play-state:{paused
+				? 'paused'
+				: 'running'}"
+		></span>
+	{/each}
+</div>