refactor(discord): remove rich presence and bundled SDK

Remove the presence setting, platform bridge, SDK archive, and SDK preparation from local and CI builds. Reject leftover Discord native libraries in release APK audits.

lib/models/settings.dart

@@ -32,7 +32,6 @@ class AppSettings {
   // Apply ReplayGain/R128 tags as volume normalization in the built-in player.
   final bool playbackNormalization;
   final bool autoMix;
-  final bool discordRichPresence;
   final bool isFirstLaunch;
   final bool checkForUpdates;
   final String updateChannel;
@@ -145,7 +144,6 @@ class AppSettings {
     this.embedReplayGain = false,
     this.playbackNormalization = false,
     this.autoMix = false,
-    this.discordRichPresence = false,
     this.isFirstLaunch = true,
     this.checkForUpdates = true,
     this.updateChannel = 'stable',
@@ -233,7 +231,6 @@ class AppSettings {
     bool? embedReplayGain,
     bool? playbackNormalization,
     bool? autoMix,
-    bool? discordRichPresence,
     bool? isFirstLaunch,
     bool? checkForUpdates,
     String? updateChannel,
@@ -317,7 +314,6 @@ class AppSettings {
       playbackNormalization:
           playbackNormalization ?? this.playbackNormalization,
       autoMix: autoMix ?? this.autoMix,
-      discordRichPresence: discordRichPresence ?? this.discordRichPresence,
       isFirstLaunch: isFirstLaunch ?? this.isFirstLaunch,
       checkForUpdates: checkForUpdates ?? this.checkForUpdates,
       updateChannel: updateChannel ?? this.updateChannel,

lib/models/settings.g.dart

@@ -23,7 +23,6 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   embedReplayGain: json['embedReplayGain'] as bool? ?? false,
   playbackNormalization: json['playbackNormalization'] as bool? ?? false,
   autoMix: json['autoMix'] as bool? ?? false,
-  discordRichPresence: json['discordRichPresence'] as bool? ?? false,
   isFirstLaunch: json['isFirstLaunch'] as bool? ?? true,
   checkForUpdates: json['checkForUpdates'] as bool? ?? true,
   updateChannel: json['updateChannel'] as String? ?? 'stable',
@@ -123,7 +122,6 @@ Map<String, dynamic> _$AppSettingsToJson(
   'embedReplayGain': instance.embedReplayGain,
   'playbackNormalization': instance.playbackNormalization,
   'autoMix': instance.autoMix,
-  'discordRichPresence': instance.discordRichPresence,
   'isFirstLaunch': instance.isFirstLaunch,
   'checkForUpdates': instance.checkForUpdates,
   'updateChannel': instance.updateChannel,