feat: add sound and vibration feedback for registration actions

Integrate audioplayers, vibration, and file_picker packages to provide
audio/haptic feedback on registration success/failure. Add sound
settings UI in SettingsPage for customizing notification sounds.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/services/sound_service.dart

@@ -0,0 +1,59 @@
+import 'package:audioplayers/audioplayers.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+class SoundService {
+  static const _keySuccess = 'sound_success';
+  static const _keyFailure = 'sound_failure';
+
+  final _player = AudioPlayer();
+
+  Future<String?> getSuccessPath() async {
+    final prefs = await SharedPreferences.getInstance();
+    return prefs.getString(_keySuccess);
+  }
+
+  Future<String?> getFailurePath() async {
+    final prefs = await SharedPreferences.getInstance();
+    return prefs.getString(_keyFailure);
+  }
+
+  Future<void> setSuccessPath(String? path) async {
+    final prefs = await SharedPreferences.getInstance();
+    if (path != null) {
+      await prefs.setString(_keySuccess, path);
+    } else {
+      await prefs.remove(_keySuccess);
+    }
+  }
+
+  Future<void> setFailurePath(String? path) async {
+    final prefs = await SharedPreferences.getInstance();
+    if (path != null) {
+      await prefs.setString(_keyFailure, path);
+    } else {
+      await prefs.remove(_keyFailure);
+    }
+  }
+
+  Future<void> playSuccess() async {
+    final path = await getSuccessPath();
+    if (path != null) {
+      await _player.play(DeviceFileSource(path));
+    }
+  }
+
+  Future<void> playFailure() async {
+    final path = await getFailurePath();
+    if (path != null) {
+      await _player.play(DeviceFileSource(path));
+    }
+  }
+
+  Future<void> play(String path) async {
+    await _player.play(DeviceFileSource(path));
+  }
+
+  Future<void> dispose() async {
+    await _player.dispose();
+  }
+}