perf(player): batch addAll/playAllNext to avoid cascading reactivity

addAll() now collects all new tracks and pushes them in a single
array operation instead of calling add() in a loop, which triggered
per-item reactive updates and O(n²) existence checks.

src/lib/player/store.svelte.ts

@@ -169,8 +169,26 @@ class PlayerStore {
 	}
 
 	addAll(tracks: Track[]) {
+		// Batch: collect new tracks, push all at once
+		const newTracks: Track[] = [];
 		for (const track of tracks) {
-			this.add(track);
+			// Check existence inline to avoid O(n) per-track via add()
+			if (!this.queue.some((t) => t.id === track.id)) {
+				newTracks.push(track);
+			}
+		}
+		if (newTracks.length === 0) return;
+
+		const startIdx = this.queue.length;
+		this.queue.push(...newTracks);
+
+		if (this.isShuffled) {
+			const newIndices = newTracks.map((_, i) => startIdx + i);
+			this.shuffledIndices.push(...newIndices);
+		}
+
+		if (startIdx === 0 && !this.currentId) {
+			this.currentId = newTracks[0].id;
 		}
 	}