feat(library): pause and resume library scans

The scan progress tile offers Pause and Resume. A paused scan releases its foreground service and wake lock, shows a paused notice, and resumes from the next unscanned file.

test/library_scan_pause_test.dart

@@ -0,0 +1,110 @@
+import 'package:flutter/services.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spotiflac_android/providers/local_library_provider.dart';
+
+class _ScanningLibrary extends LocalLibraryNotifier {
+  _ScanningLibrary({this.finalizing = false});
+
+  final bool finalizing;
+
+  @override
+  LocalLibraryState build() => LocalLibraryState(
+    isScanning: true,
+    scanIsFinalizing: finalizing,
+    scannedFiles: 100,
+    scanTotalFiles: 400,
+    scanProgress: 25,
+  );
+}
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  const backend = MethodChannel('com.zarz.spotiflac/backend');
+  final messenger =
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger;
+  late List<String> calls;
+
+  setUp(() {
+    calls = <String>[];
+    messenger.setMockMethodCallHandler(backend, (call) async {
+      calls.add(call.method);
+      return null;
+    });
+  });
+  tearDown(() => messenger.setMockMethodCallHandler(backend, null));
+
+  ProviderContainer containerFor(_ScanningLibrary library) {
+    final container = ProviderContainer(
+      overrides: [localLibraryProvider.overrideWith(() => library)],
+    );
+    addTearDown(container.dispose);
+    return container;
+  }
+
+  test('pause holds the scan in place and resume continues it', () async {
+    final container = containerFor(_ScanningLibrary());
+    final notifier = container.read(localLibraryProvider.notifier);
+
+    await notifier.pauseScan();
+    var state = container.read(localLibraryProvider);
+    expect(state.isScanning, isTrue);
+    expect(state.scanIsPaused, isTrue);
+    expect(state.scannedFiles, 100);
+
+    // Repeated pauses do not reach native code again.
+    await notifier.pauseScan();
+    expect(calls, ['pauseLibraryScan']);
+
+    await notifier.resumeScan();
+    state = container.read(localLibraryProvider);
+    expect(state.isScanning, isTrue);
+    expect(state.scanIsPaused, isFalse);
+    expect(state.scannedFiles, 100);
+    expect(calls, ['pauseLibraryScan', 'resumeLibraryScan']);
+
+    await notifier.resumeScan();
+    expect(calls, ['pauseLibraryScan', 'resumeLibraryScan']);
+  });
+
+  test('cancel releases a paused scan without resuming it first', () async {
+    final container = containerFor(_ScanningLibrary());
+    final notifier = container.read(localLibraryProvider.notifier);
+
+    await notifier.pauseScan();
+    await notifier.cancelScan();
+
+    final state = container.read(localLibraryProvider);
+    expect(state.scanIsPaused, isFalse);
+    expect(state.scanWasCancelled, isTrue);
+    expect(calls, ['pauseLibraryScan', 'cancelLibraryScan']);
+
+    await notifier.resumeScan();
+    expect(calls, ['pauseLibraryScan', 'cancelLibraryScan']);
+  });
+
+  test('finalizing scans cannot be paused', () async {
+    final container = containerFor(_ScanningLibrary(finalizing: true));
+    final notifier = container.read(localLibraryProvider.notifier);
+
+    await notifier.pauseScan();
+
+    expect(container.read(localLibraryProvider).scanIsPaused, isFalse);
+    expect(calls, isEmpty);
+  });
+
+  test('a failed native pause leaves the scan running', () async {
+    messenger.setMockMethodCallHandler(backend, (call) async {
+      calls.add(call.method);
+      throw PlatformException(code: 'pause-unavailable');
+    });
+    final container = containerFor(_ScanningLibrary());
+    final notifier = container.read(localLibraryProvider.notifier);
+
+    await notifier.pauseScan();
+
+    expect(container.read(localLibraryProvider).scanIsPaused, isFalse);
+    await notifier.resumeScan();
+    expect(calls, ['pauseLibraryScan']);
+  });
+}