refactor(web): share track target helpers between home and explore

Both pages reimplemented the same accessors over the target union and a
byte-identical queue-item mapper. Moves the domain type (renamed
ContextMenuTarget -> TrackTarget, since tiles and the queue use it too)
and its helpers to $lib/tracks.ts; TrackContextMenu keeps only its menu
geometry. Home drops from 341 to ~230 lines.

Unifies two divergences the duplication was hiding: home compared the
active tile against track.id while the queue is keyed by
queueIdForTrack, so YouTube-sourced tracks never highlighted; and it
hardcoded explicit to false for local tracks, hiding the badge.

Also cancels in-flight youtube filler results when the effect re-runs,
so a slow response cannot overwrite newer ones.

frontend/src/lib/components/TrackContextMenu.svelte

@@ -1,20 +1,12 @@
 <script lang="ts" module>
-	import type { ApiTrackLike } from '$lib/player/player.svelte';
-	import type { YouTubeSong } from '$lib/types';
+	import type { TrackTarget } from '$lib/tracks';
 
-	export type ContextMenuTarget =
-		| { kind: 'youtube'; song: YouTubeSong }
-		| { kind: 'local'; track: ApiTrackLike };
-
-	export type ContextMenuState = ContextMenuTarget & { x: number; y: number; openLeft: boolean };
+	export type ContextMenuState = TrackTarget & { x: number; y: number; openLeft: boolean };
 
 	const MENU_WIDTH = 208;
 	const SUBMENU_WIDTH = 224;
 
-	export function contextMenuStateFor(
-		event: MouseEvent,
-		target: ContextMenuTarget
-	): ContextMenuState {
+	export function contextMenuStateFor(event: MouseEvent, target: TrackTarget): ContextMenuState {
 		event.preventDefault();
 		return {
 			...target,

frontend/src/lib/tracks.ts

@@ -0,0 +1,79 @@
+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
+				}
+			};
+}