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/screens/settings/library_settings_page.dart

@@ -9,6 +9,7 @@ import 'package:spotiflac_android/models/settings.dart';
 import 'package:spotiflac_android/providers/settings_provider.dart';
 import 'package:spotiflac_android/providers/local_library_provider.dart';
 import 'package:spotiflac_android/services/library_database.dart';
+import 'package:spotiflac_android/services/discord_presence_service.dart';
 import 'package:spotiflac_android/services/platform_bridge.dart';
 import 'package:spotiflac_android/utils/adaptive_layout.dart';
 import 'package:spotiflac_android/widgets/duplicate_review_sheet.dart';
@@ -910,8 +911,27 @@ class _LibrarySettingsPageState extends ConsumerState<LibrarySettingsPage> {
                   onChanged: (v) => ref
                       .read(settingsProvider.notifier)
                       .setPlaybackNormalization(v),
-                  showDivider: false,
+                  showDivider: Platform.isAndroid,
                 ),
+                if (Platform.isAndroid)
+                  ValueListenableBuilder<String>(
+                    valueListenable: DiscordPresenceService.instance.status,
+                    builder: (context, status, _) => SettingsSwitchItem(
+                      icon: Icons.share_outlined,
+                      title: context.l10n.discordRichPresence,
+                      subtitle: status == 'discord_missing'
+                          ? context.l10n.discordPresenceMissing
+                          : status == 'sdk_unavailable' ||
+                                status == 'unavailable'
+                          ? context.l10n.discordPresenceUnavailable
+                          : context.l10n.discordPresenceDescription,
+                      value: settings.discordRichPresence,
+                      onChanged: (value) => ref
+                          .read(settingsProvider.notifier)
+                          .setDiscordRichPresence(value),
+                      showDivider: false,
+                    ),
+                  ),
               ],
             ),
           ),