Move domain data helpers into lib/data

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web-player/src/lib/data/mixes.ts

@@ -0,0 +1,18 @@
+import type { components } from '$lib/api/schema';
+import type { ApiTrackLike } from '$lib/player/player.svelte';
+
+export type Mix = components['schemas']['MixApplicationResponse'];
+export type MixItem = components['schemas']['MixItemApplicationResponse'];
+
+export function mixItemKey(item: MixItem): string {
+	return `mf:${item.trackId}`;
+}
+
+export function mixItemTrack(item: MixItem): ApiTrackLike {
+	return {
+		id: item.trackId,
+		title: item.title,
+		artist: item.artist,
+		listensCount: item.listensCount
+	};
+}