Implement 4-step mosaic flow, robust project autosave, and JSON export

test/project_codec_test.dart

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+import 'dart:typed_data';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:korken_mosaic/project_codec.dart';
+
+void main() {
+  test('MosaicProjectData json roundtrip keeps values', () {
+    final original = MosaicProjectData(
+      useCapSize: true,
+      gridWidth: '50',
+      gridHeight: '40',
+      capSize: '10',
+      fidelityStructure: 0.3,
+      ditheringStrength: 0.2,
+      edgeEmphasis: 0.4,
+      colorVariation: 0.5,
+      selectedPreset: 'realistisch',
+      sourceImageBytes: Uint8List.fromList([1, 2, 3]),
+      savedAt: DateTime.parse('2026-01-01T12:00:00Z'),
+    );
+
+    final decoded = MosaicProjectData.fromJson(original.toJson());
+
+    expect(decoded.useCapSize, isTrue);
+    expect(decoded.gridWidth, '50');
+    expect(decoded.gridHeight, '40');
+    expect(decoded.capSize, '10');
+    expect(decoded.selectedPreset, 'realistisch');
+    expect(decoded.sourceImageBytes, isNotNull);
+    expect(decoded.sourceImageBytes!, [1, 2, 3]);
+  });
+}

test/widget_test.dart

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+import 'package:flutter_test/flutter_test.dart';
+import 'package:korken_mosaic/main.dart';
+
+void main() {
+  testWidgets('shows 4-step workflow and can navigate to result step',
+      (WidgetTester tester) async {
+    await tester.pumpWidget(const KorkenMosaicApp());
+
+    expect(find.text('1) Bild'), findsOneWidget);
+    expect(find.text('2) Größe'), findsOneWidget);
+    expect(find.text('3) Farben'), findsOneWidget);
+    expect(find.text('4) Ergebnis'), findsOneWidget);
+
+    await tester.tap(find.text('Weiter'));
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('Weiter'));
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('Weiter'));
+    await tester.pumpAndSettle();
+
+    expect(find.byKey(const Key('generate-btn')), findsOneWidget);
+    expect(find.text('Export JSON'), findsOneWidget);
+  });
+}