feat: add opt-in Discord Rich Presence on Android

Use the official Discord Social SDK through JNI to share built-in player activity, clear presence when playback stops, and guard against late callbacks after disabling. Package the SDK optionally through SPOTIFLAC_DISCORD_SDK_DIR with license notices.

Credit: @itsmegaaa for the Rich Presence initiative and contribution in #575 and #576.

Validation: 15 Dart tests, scoped Flutter analysis, and one Android 16 ARM64 SDK/JNI instrumentation test passed. Visible presence on a signed-in Discord profile remains unverified.

lib/models/settings.g.dart

@@ -22,6 +22,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   embedLyrics: json['embedLyrics'] as bool? ?? true,
   embedReplayGain: json['embedReplayGain'] as bool? ?? false,
   playbackNormalization: json['playbackNormalization'] as bool? ?? false,
+  discordRichPresence: json['discordRichPresence'] as bool? ?? false,
   isFirstLaunch: json['isFirstLaunch'] as bool? ?? true,
   checkForUpdates: json['checkForUpdates'] as bool? ?? true,
   updateChannel: json['updateChannel'] as String? ?? 'stable',
@@ -117,6 +118,7 @@ Map<String, dynamic> _$AppSettingsToJson(
   'embedLyrics': instance.embedLyrics,
   'embedReplayGain': instance.embedReplayGain,
   'playbackNormalization': instance.playbackNormalization,
+  'discordRichPresence': instance.discordRichPresence,
   'isFirstLaunch': instance.isFirstLaunch,
   'checkForUpdates': instance.checkForUpdates,
   'updateChannel': instance.updateChannel,