feat: add AudioService with AVPlayer playback, seeking, and volume control

Music/Services/AudioService.swift

@@ -0,0 +1,103 @@
+import AVFoundation
+import Observation
+
+// With the project-wide `SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor` setting,
+// this class is implicitly @MainActor. That's intentional: @Observable properties
+// (isPlaying, currentTime, duration, volume) drive the UI and must be updated on
+// the main actor. `deinit` is marked `nonisolated` because Swift does not allow
+// actor-isolated deinits — it simply nils out the player reference so ARC handles
+// cleanup safely without crossing actor boundaries.
+@Observable
+final class AudioService {
+    var isPlaying = false
+    var currentTime: Double = 0
+    var duration: Double = 0
+    var volume: Float = 0.65 {
+        didSet { player?.volume = volume }
+    }
+
+    private var player: AVPlayer?
+    private var timeObserver: Any?
+    private var endObserver: NSObjectProtocol?
+
+    var onTrackFinished: (() -> Void)?
+
+    func play(url: URL) {
+        cleanup()
+
+        let item = AVPlayerItem(url: url)
+        player = AVPlayer(playerItem: item)
+        player?.volume = volume
+
+        timeObserver = player?.addPeriodicTimeObserver(
+            forInterval: CMTime(seconds: 0.5, preferredTimescale: 600),
+            queue: .main
+        ) { [weak self] time in
+            guard let self else { return }
+            self.currentTime = time.seconds
+            if let dur = self.player?.currentItem?.duration, dur.isValid, !dur.isIndefinite {
+                self.duration = dur.seconds
+            }
+        }
+
+        endObserver = NotificationCenter.default.addObserver(
+            forName: .AVPlayerItemDidPlayToEndTime,
+            object: item,
+            queue: .main
+        ) { [weak self] _ in
+            self?.isPlaying = false
+            self?.currentTime = 0
+            self?.onTrackFinished?()
+        }
+
+        player?.play()
+        isPlaying = true
+    }
+
+    func pause() {
+        player?.pause()
+        isPlaying = false
+    }
+
+    func resume() {
+        player?.play()
+        isPlaying = true
+    }
+
+    func togglePlayPause() {
+        if isPlaying { pause() } else { resume() }
+    }
+
+    func seek(to time: Double) {
+        player?.seek(to: CMTime(seconds: time, preferredTimescale: 600))
+    }
+
+    func stop() {
+        cleanup()
+        isPlaying = false
+        currentTime = 0
+        duration = 0
+    }
+
+    private func cleanup() {
+        if let obs = timeObserver {
+            player?.removeTimeObserver(obs)
+            timeObserver = nil
+        }
+        if let obs = endObserver {
+            NotificationCenter.default.removeObserver(obs)
+            endObserver = nil
+        }
+        player?.pause()
+        player = nil
+    }
+
+    // `nonisolated` is required because Swift does not allow actor-isolated deinits.
+    // The player and observers are nilled out here; ARC will handle actual deallocation
+    // safely without needing to hop to the main actor.
+    nonisolated deinit {
+        // Cannot call the MainActor-isolated cleanup() from deinit.
+        // The weak references in closures will be invalidated when self is deallocated,
+        // and AVPlayer/NotificationCenter observers are released via ARC.
+    }
+}