feat(discord): add opt-in mobile rich presence with track artwork

lib/models/settings.dart

@@ -32,6 +32,7 @@ class AppSettings {
   // Apply ReplayGain/R128 tags as volume normalization in the built-in player.
   final bool playbackNormalization;
   final bool autoMix;
+  final bool discordRichPresenceEnabled;
   final bool usbBitPerfect;
   final bool usbDirect;
   final bool usbDsdOverPcm;
@@ -153,6 +154,7 @@ class AppSettings {
     this.embedReplayGain = false,
     this.playbackNormalization = false,
     this.autoMix = false,
+    this.discordRichPresenceEnabled = false,
     this.usbBitPerfect = false,
     this.usbDirect = false,
     this.usbDsdOverPcm = false,
@@ -249,6 +251,7 @@ class AppSettings {
     bool? embedReplayGain,
     bool? playbackNormalization,
     bool? autoMix,
+    bool? discordRichPresenceEnabled,
     bool? usbBitPerfect,
     bool? usbDirect,
     bool? usbDsdOverPcm,
@@ -341,6 +344,8 @@ class AppSettings {
       playbackNormalization:
           playbackNormalization ?? this.playbackNormalization,
       autoMix: autoMix ?? this.autoMix,
+      discordRichPresenceEnabled:
+          discordRichPresenceEnabled ?? this.discordRichPresenceEnabled,
       usbBitPerfect: usbBitPerfect ?? this.usbBitPerfect,
       usbDirect: usbDirect ?? this.usbDirect,
       usbDsdOverPcm: usbDsdOverPcm ?? this.usbDsdOverPcm,

lib/models/settings.g.dart

@@ -23,6 +23,8 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   embedReplayGain: json['embedReplayGain'] as bool? ?? false,
   playbackNormalization: json['playbackNormalization'] as bool? ?? false,
   autoMix: json['autoMix'] as bool? ?? false,
+  discordRichPresenceEnabled:
+      json['discordRichPresenceEnabled'] as bool? ?? false,
   usbBitPerfect: json['usbBitPerfect'] as bool? ?? false,
   usbDirect: json['usbDirect'] as bool? ?? false,
   usbDsdOverPcm: json['usbDsdOverPcm'] as bool? ?? false,
@@ -132,6 +134,7 @@ Map<String, dynamic> _$AppSettingsToJson(
   'embedReplayGain': instance.embedReplayGain,
   'playbackNormalization': instance.playbackNormalization,
   'autoMix': instance.autoMix,
+  'discordRichPresenceEnabled': instance.discordRichPresenceEnabled,
   'usbBitPerfect': instance.usbBitPerfect,
   'usbDirect': instance.usbDirect,
   'usbDsdOverPcm': instance.usbDsdOverPcm,