feat(project): persist catalog snapshot in project data

test/project_codec_test.dart

@@ -4,7 +4,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:korken_mosaic/project_codec.dart';
 
 void main() {
-  test('MosaicProjectData json roundtrip keeps values', () {
+  test('MosaicProjectData json roundtrip keeps values including catalog snapshot', () {
     final original = MosaicProjectData(
       useCapSize: true,
       gridWidth: '50',
@@ -16,6 +16,10 @@ void main() {
       colorVariation: 0.5,
       selectedPreset: 'realistisch',
       sourceImageBytes: Uint8List.fromList([1, 2, 3]),
+      catalogSnapshot: const [
+        MosaicPaletteSnapshotEntry(name: 'White', colorValue: 0xFFF2F2F2),
+        MosaicPaletteSnapshotEntry(name: 'Blue', colorValue: 0xFF3F6FD8),
+      ],
       savedAt: DateTime.parse('2026-01-01T12:00:00Z'),
     );
 
@@ -28,5 +32,22 @@ void main() {
     expect(decoded.selectedPreset, 'realistisch');
     expect(decoded.sourceImageBytes, isNotNull);
     expect(decoded.sourceImageBytes!, [1, 2, 3]);
+    expect(decoded.catalogSnapshot.length, 2);
+    expect(decoded.catalogSnapshot.first.name, 'White');
+    expect(decoded.catalogSnapshot.last.colorValue, 0xFF3F6FD8);
+  });
+
+  test('MosaicProjectData defaults to empty snapshot when old project has none', () {
+    final decoded = MosaicProjectData.fromJson({
+      'useCapSize': false,
+      'gridWidth': '40',
+      'gridHeight': '30',
+      'capSize': '12',
+      'selectedPreset': 'ausgewogen',
+      'savedAt': '2026-01-01T12:00:00Z',
+    });
+
+    expect(decoded.catalogSnapshot, isEmpty);
+    expect(decoded.sourceImageBytes, isNull);
   });
 }