feat(project): persist catalog snapshot in project data

lib/project_codec.dart

@@ -1,6 +1,28 @@
 import 'dart:convert';
 import 'dart:typed_data';
 
+class MosaicPaletteSnapshotEntry {
+  final String name;
+  final int colorValue;
+
+  const MosaicPaletteSnapshotEntry({
+    required this.name,
+    required this.colorValue,
+  });
+
+  Map<String, dynamic> toJson() => {
+        'name': name,
+        'colorValue': colorValue,
+      };
+
+  factory MosaicPaletteSnapshotEntry.fromJson(Map<String, dynamic> json) {
+    return MosaicPaletteSnapshotEntry(
+      name: json['name'] as String? ?? 'Unbenannt',
+      colorValue: (json['colorValue'] as num?)?.toInt() ?? 0xFF000000,
+    );
+  }
+}
+
 class MosaicProjectData {
   final bool useCapSize;
   final String gridWidth;
@@ -12,6 +34,7 @@ class MosaicProjectData {
   final double colorVariation;
   final String selectedPreset;
   final Uint8List? sourceImageBytes;
+  final List<MosaicPaletteSnapshotEntry> catalogSnapshot;
   final DateTime savedAt;
 
   const MosaicProjectData({
@@ -25,6 +48,7 @@ class MosaicProjectData {
     required this.colorVariation,
     required this.selectedPreset,
     required this.sourceImageBytes,
+    required this.catalogSnapshot,
     required this.savedAt,
   });
 
@@ -40,23 +64,32 @@ class MosaicProjectData {
         'selectedPreset': selectedPreset,
         'sourceImageBase64':
             sourceImageBytes == null ? null : base64Encode(sourceImageBytes!),
+        'catalogSnapshot': catalogSnapshot.map((entry) => entry.toJson()).toList(),
         'savedAt': savedAt.toIso8601String(),
       };
 
   factory MosaicProjectData.fromJson(Map<String, dynamic> json) {
     final sourceB64 = json['sourceImageBase64'] as String?;
+    final snapshotRaw = (json['catalogSnapshot'] as List?) ?? const [];
     return MosaicProjectData(
       useCapSize: json['useCapSize'] as bool? ?? false,
       gridWidth: json['gridWidth'] as String? ?? '40',
       gridHeight: json['gridHeight'] as String? ?? '30',
       capSize: json['capSize'] as String? ?? '12',
       fidelityStructure: (json['fidelityStructure'] as num?)?.toDouble() ?? 0.5,
-      ditheringStrength: (json['ditheringStrength'] as num?)?.toDouble() ?? 0.35,
+      ditheringStrength:
+          (json['ditheringStrength'] as num?)?.toDouble() ?? 0.35,
       edgeEmphasis: (json['edgeEmphasis'] as num?)?.toDouble() ?? 0.4,
       colorVariation: (json['colorVariation'] as num?)?.toDouble() ?? 0.3,
       selectedPreset: json['selectedPreset'] as String? ?? 'ausgewogen',
       sourceImageBytes: sourceB64 == null ? null : base64Decode(sourceB64),
-      savedAt: DateTime.tryParse(json['savedAt'] as String? ?? '') ?? DateTime.now(),
+      catalogSnapshot: snapshotRaw
+          .whereType<Map>()
+          .map((entry) => MosaicPaletteSnapshotEntry.fromJson(
+              Map<String, dynamic>.from(entry)))
+          .toList(growable: false),
+      savedAt:
+          DateTime.tryParse(json['savedAt'] as String? ?? '') ?? DateTime.now(),
     );
   }
 }

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);
   });
 }