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/main_shell.dart

@@ -7,6 +7,7 @@ import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 import 'package:spotiflac_android/l10n/l10n.dart';
+import 'package:spotiflac_android/services/discord_presence_service.dart';
 import 'package:spotiflac_android/providers/download_queue_provider.dart';
 import 'package:spotiflac_android/providers/settings_provider.dart';
 import 'package:spotiflac_android/providers/repo_provider.dart';
@@ -79,6 +80,11 @@ class _MainShellState extends ConsumerState<MainShell>
     setPlaybackNormalizationEnabled(
       ref.read(settingsProvider).playbackNormalization,
     );
+    unawaited(
+      DiscordPresenceService.instance.setEnabled(
+        ref.read(settingsProvider).discordRichPresence,
+      ),
+    );
     // Deezer & co. localize artist/genre names by IP unless told the app's
     // language (issue #480).
     unawaited(
@@ -631,6 +637,12 @@ class _MainShellState extends ConsumerState<MainShell>
 
   @override
   Widget build(BuildContext context) {
+    ref.listen(settingsProvider.select((s) => s.discordRichPresence), (
+      _,
+      enabled,
+    ) {
+      unawaited(DiscordPresenceService.instance.setEnabled(enabled));
+    });
     ref.listen(settingsProvider.select((s) => s.playbackNormalization), (
       _,
       enabled,