perf(player): add idToIndex Map for O(1) track lookups

Replace all linear find/findIndex/some scans with a derived Map
keyed by track.id. This makes currentTrack, currentIndex, hasTrack,
add, playNext, remove, and addAll all O(1) for ID lookups.

src/lib/player/store.svelte.ts

@@ -26,16 +26,26 @@ class PlayerStore {
 	isMuted = $state(false);
 	uiOpen = $state(false); // Mobile UI state
 
-	// Derived
-	currentTrack = $derived(
-		this.currentId
-			? (this.queue.find((t) => t.id === this.currentId) ?? null)
-			: null,
-	);
+	// O(1) index: track.id → index in queue
+	private idToIndex = $derived.by(() => {
+		const map = new Map<number, number>();
+		for (let i = 0; i < this.queue.length; i++) {
+			map.set(this.queue[i].id, i);
+		}
+		return map;
+	});
 
-	currentIndex = $derived(
-		this.currentId ? this.queue.findIndex((t) => t.id === this.currentId) : -1,
-	);
+	// Derived
+	currentTrack = $derived.by(() => {
+		if (this.currentId == null) return null;
+		const idx = this.idToIndex.get(this.currentId);
+		return idx !== undefined ? this.queue[idx] : null;
+	});
+
+	currentIndex = $derived.by(() => {
+		if (this.currentId == null) return -1;
+		return this.idToIndex.get(this.currentId) ?? -1;
+	});
 
 	displayQueue = $derived(
 		this.isShuffled
@@ -46,7 +56,7 @@ class PlayerStore {
 	);
 
 	hasTrack(id: number) {
-		return this.queue.some((t) => t.id === id);
+		return this.idToIndex.has(id);
 	}
 
 	constructor() {
@@ -105,9 +115,9 @@ class PlayerStore {
 	// Actions
 
 	add(track: Track, playNow = false) {
-		const existingIdx = this.queue.findIndex((t) => t.id === track.id);
+		const exists = this.hasTrack(track.id);
 
-		if (existingIdx !== -1) {
+		if (exists) {
 			if (playNow) {
 				this.playNext(track);
 				this.playId(track.id);
@@ -133,7 +143,7 @@ class PlayerStore {
 	}
 
 	playNext(track: Track) {
-		const existingIdx = this.queue.findIndex((t) => t.id === track.id);
+		const existingIdx = this.idToIndex.get(track.id) ?? -1;
 		const targetTrack = track;
 
 		if (existingIdx !== -1) {
@@ -173,7 +183,7 @@ class PlayerStore {
 		const newTracks: Track[] = [];
 		for (const track of tracks) {
 			// Check existence inline to avoid O(n) per-track via add()
-			if (!this.queue.some((t) => t.id === track.id)) {
+			if (!this.hasTrack(track.id)) {
 				newTracks.push(track);
 			}
 		}
@@ -200,8 +210,8 @@ class PlayerStore {
 	}
 
 	remove(id: number) {
-		const idx = this.queue.findIndex((t) => t.id === id);
-		if (idx === -1) return;
+		const idx = this.idToIndex.get(id);
+		if (idx === undefined) return;
 
 		const wasCurrent = this.currentId === id;