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

lib/project_codec.dart

@@ -0,0 +1,62 @@
+import 'dart:convert';
+import 'dart:typed_data';
+
+class MosaicProjectData {
+  final bool useCapSize;
+  final String gridWidth;
+  final String gridHeight;
+  final String capSize;
+  final double fidelityStructure;
+  final double ditheringStrength;
+  final double edgeEmphasis;
+  final double colorVariation;
+  final String selectedPreset;
+  final Uint8List? sourceImageBytes;
+  final DateTime savedAt;
+
+  const MosaicProjectData({
+    required this.useCapSize,
+    required this.gridWidth,
+    required this.gridHeight,
+    required this.capSize,
+    required this.fidelityStructure,
+    required this.ditheringStrength,
+    required this.edgeEmphasis,
+    required this.colorVariation,
+    required this.selectedPreset,
+    required this.sourceImageBytes,
+    required this.savedAt,
+  });
+
+  Map<String, dynamic> toJson() => {
+        'useCapSize': useCapSize,
+        'gridWidth': gridWidth,
+        'gridHeight': gridHeight,
+        'capSize': capSize,
+        'fidelityStructure': fidelityStructure,
+        'ditheringStrength': ditheringStrength,
+        'edgeEmphasis': edgeEmphasis,
+        'colorVariation': colorVariation,
+        'selectedPreset': selectedPreset,
+        'sourceImageBase64':
+            sourceImageBytes == null ? null : base64Encode(sourceImageBytes!),
+        'savedAt': savedAt.toIso8601String(),
+      };
+
+  factory MosaicProjectData.fromJson(Map<String, dynamic> json) {
+    final sourceB64 = json['sourceImageBase64'] as String?;
+    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,
+      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(),
+    );
+  }
+}