refactor: rename frontend/ to web-player/ and format with prettier

Updates the compose build context, the Dockerfile header, the package
name and the docs. The compose service and container keep the name
frontend/musify-frontend, since nginx routes to http://frontend:3000.

The source files also carry the first prettier pass, so the rename and
the reformat land together for them.

web-player/src/lib/tracks.ts

@@ -0,0 +1,78 @@
+import {
+	player,
+	queueIdForTrack,
+	toQueueItems,
+	type ApiTrackLike,
+	type QueueItem
+} from '$lib/player/player.svelte';
+import type { YouTubeSong } from '$lib/types';
+
+export type TrackTarget =
+	{ kind: 'youtube'; song: YouTubeSong } | { kind: 'local'; track: ApiTrackLike };
+
+export function targetId(target: TrackTarget): string | number {
+	return target.kind === 'youtube' ? target.song.videoId : target.track.id;
+}
+
+export function targetTitle(target: TrackTarget): string {
+	return target.kind === 'youtube' ? target.song.title : target.track.title;
+}
+
+export function targetArtist(target: TrackTarget): string | undefined {
+	return target.kind === 'youtube' ? target.song.artist : target.track.artist;
+}
+
+export function targetExplicit(target: TrackTarget): boolean {
+	return target.kind === 'youtube' ? target.song.isExplicit : (target.track.isExplicit ?? false);
+}
+
+export function targetCoverSrc(target: TrackTarget): string | undefined {
+	return target.kind === 'youtube' ? target.song.thumbnailUrl : undefined;
+}
+
+export function queueIdForTarget(target: TrackTarget): string | number {
+	return target.kind === 'youtube' ? target.song.videoId : queueIdForTrack(target.track);
+}
+
+export function isTargetCurrent(target: TrackTarget): boolean {
+	return player.current.id === queueIdForTarget(target);
+}
+
+export function queueItemForTarget(target: TrackTarget): QueueItem {
+	return target.kind === 'youtube'
+		? {
+				id: target.song.videoId,
+				title: target.song.title,
+				artist: target.song.artist,
+				source: 'youtube',
+				coverUrl: target.song.thumbnailUrl,
+				explicit: target.song.isExplicit
+			}
+		: toQueueItems([target.track])[0];
+}
+
+export function targetForQueueItem(item: QueueItem): TrackTarget {
+	return item.source === 'youtube'
+		? {
+				kind: 'youtube',
+				song: {
+					videoId: String(item.id),
+					title: item.title,
+					artist: item.artist ?? '',
+					album: '',
+					durationSeconds: 0,
+					thumbnailUrl: item.coverUrl ?? '',
+					isExplicit: item.explicit ?? false
+				}
+			}
+		: {
+				kind: 'local',
+				track: {
+					id: item.id,
+					title: item.title,
+					artist: item.artist,
+					source: 'Local',
+					isExplicit: item.explicit
+				}
+			};
+}