feat: show remote-config launch announcement on app start

Introduce AppRemoteConfigService which fetches a platform/version/locale-aware JSON payload from api.zarz.moe/v1/spotiflac-mobile/config and caches it in SharedPreferences. main_shell shows a one-shot announcement dialog (respecting dismissible, CTA, time window and version gates) when no update prompt is pending; dismissed IDs are persisted so each announcement surfaces only once.

Tweaks bundled in: the service health dot loses its blur halo in favour of solid Material 3 tones, and AppInfo gains the remote config endpoint constant. The share listener and SAF migration hook stay synchronous inside the post-frame callback so share-intent URLs never race the network-bound checks.

New unit tests cover the announcement CTA/active-window rules.

test/models_and_utils_test.dart

@@ -4,6 +4,7 @@ import 'package:spotiflac_android/models/download_item.dart';
 import 'package:spotiflac_android/models/settings.dart';
 import 'package:spotiflac_android/models/theme_settings.dart';
 import 'package:spotiflac_android/models/track.dart';
+import 'package:spotiflac_android/services/app_remote_config_service.dart';
 import 'package:spotiflac_android/services/download_request_payload.dart';
 import 'package:spotiflac_android/utils/artist_utils.dart';
 import 'package:spotiflac_android/utils/mime_utils.dart';
@@ -507,4 +508,127 @@ void main() {
       expect(keys, contains('C:/Music/Song'));
     });
   });
+
+  group('AppRemoteConfig', () {
+    test('parses announcement and donate payloads from API JSON', () {
+      final config = AppRemoteConfig.fromJson({
+        'announcement': {
+          'id': 'hello-2026',
+          'enabled': true,
+          'title': 'Server message',
+          'message': 'A clear message for users',
+          'cta_enabled': true,
+          'cta_label': 'Donate',
+          'cta_url': 'https://example.test/donate',
+          'starts_at': '2026-05-01T00:00:00Z',
+          'ends_at': '2026-06-01T00:00:00Z',
+          'min_version': '4.5.0',
+          'priority': 'high',
+        },
+        'donate': {
+          'enabled': true,
+          'title': 'Support SpotiFLAC Mobile',
+          'message': 'Help cover infrastructure.',
+          'methods': [
+            {
+              'id': 'kofi',
+              'title': 'Ko-fi',
+              'subtitle': 'ko-fi.com/example',
+              'url': 'https://ko-fi.com/example',
+              'icon': 'kofi',
+              'color': '#FF5E5B',
+            },
+            {
+              'id': 'wallet',
+              'title': 'USDT',
+              'subtitle': 'TRC20',
+              'wallet_address': 'T123',
+              'icon': 'wallet',
+              'color': '0xFF26A17B',
+            },
+          ],
+          'supporters': ['Alice', 'Bob'],
+          'notices': ['No paywalls'],
+        },
+      });
+
+      expect(config.announcement?.id, 'hello-2026');
+      expect(config.announcement?.hasCta, isTrue);
+      expect(
+        config.announcement?.isActive(
+          now: DateTime.utc(2026, 5, 11),
+          currentVersion: '4.5.1',
+        ),
+        isTrue,
+      );
+      expect(config.donate.title, 'Support SpotiFLAC Mobile');
+      expect(config.donate.methods, hasLength(2));
+      expect(config.donate.methods.first.color, 0xFFFF5E5B);
+      expect(config.donate.methods.last.isWallet, isTrue);
+      expect(config.donate.supporters, ['Alice', 'Bob']);
+      expect(config.donate.notices, ['No paywalls']);
+    });
+
+    test('requires enabled announcement CTA with label and url', () {
+      final disabledCta = RemoteAnnouncement.fromJson({
+        'id': 'notice',
+        'title': 'Notice',
+        'message': 'No button',
+        'cta_label': 'Open',
+        'cta_url': 'https://api.zarz.moe',
+      });
+      final missingLabel = RemoteAnnouncement.fromJson({
+        'id': 'notice',
+        'title': 'Notice',
+        'message': 'No button',
+        'cta_enabled': true,
+        'cta_url': 'https://example.test',
+      });
+      final enabledCta = RemoteAnnouncement.fromJson({
+        'id': 'notice',
+        'title': 'Notice',
+        'message': 'With button',
+        'cta_enabled': true,
+        'cta_label': 'Read More',
+        'cta_url': 'https://example.test',
+      });
+
+      expect(disabledCta.hasCta, isFalse);
+      expect(missingLabel.hasCta, isFalse);
+      expect(enabledCta.hasCta, isTrue);
+      expect(enabledCta.ctaLabel, 'Read More');
+    });
+
+    test('filters inactive announcements by window and app version', () {
+      final announcement = RemoteAnnouncement.fromJson({
+        'id': 'future',
+        'title': 'Future',
+        'message': 'Not yet',
+        'starts_at': '2026-06-01T00:00:00Z',
+        'min_version': '4.6.0',
+      });
+
+      expect(
+        announcement.isActive(
+          now: DateTime.utc(2026, 5, 11),
+          currentVersion: '4.5.1',
+        ),
+        isFalse,
+      );
+      expect(
+        announcement.isActive(
+          now: DateTime.utc(2026, 6, 2),
+          currentVersion: '4.5.1',
+        ),
+        isFalse,
+      );
+      expect(
+        announcement.isActive(
+          now: DateTime.utc(2026, 6, 2),
+          currentVersion: '4.6.0',
+        ),
+        isTrue,
+      );
+    });
+  });
 }