fix(flow): guard stepper and reset state on image-less project load

lib/main.dart

@@ -205,6 +205,31 @@ class _MosaicHomePageState extends State<MosaicHomePage>
     return File('${projectsDir.path}/latest_project.json');
   }
 
+  bool get _hasSourceImage => _sourceImageBytes != null;
+
+  bool get _canGenerate => _hasSourceImage && !_isGenerating;
+
+  int get _maxAccessibleStepIndex =>
+      _hasSourceImage ? MosaicFlowStep.result.index : MosaicFlowStep.image.index;
+
+  List<MosaicPaletteSnapshotEntry> _catalogSnapshotFromCurrentCatalog() {
+    return _catalog
+        .map((entry) => MosaicPaletteSnapshotEntry(
+              name: entry.name,
+              colorValue: entry.colorValue,
+            ))
+        .toList(growable: false);
+  }
+
+  void _showMissingImageHint() {
+    if (!mounted) return;
+    ScaffoldMessenger.of(context).showSnackBar(
+      const SnackBar(
+        content: Text('Bitte zuerst ein Bild auswählen.'),
+      ),
+    );
+  }
+
   MosaicProjectData _buildProjectData() {
     return MosaicProjectData(
       useCapSize: _useCapSize,
@@ -217,6 +242,7 @@ class _MosaicHomePageState extends State<MosaicHomePage>
       colorVariation: _colorVariation,
       selectedPreset: _selectedPreset.name,
       sourceImageBytes: _sourceImageBytes,
+      catalogSnapshot: _catalogSnapshotFromCurrentCatalog(),
       savedAt: DateTime.now(),
     );
   }
@@ -288,6 +314,7 @@ class _MosaicHomePageState extends State<MosaicHomePage>
       final data = MosaicProjectData.fromJson(
         jsonDecode(await file.readAsString()) as Map<String, dynamic>,
       );
+      final hasCatalogSnapshot = data.catalogSnapshot.isNotEmpty;
 
       if (!mounted) return;
       setState(() {
@@ -305,18 +332,22 @@ class _MosaicHomePageState extends State<MosaicHomePage>
           orElse: () => StylePreset.ausgewogen,
         );
 
-        if (data.sourceImageBytes != null) {
         _sourceImageBytes = data.sourceImageBytes;
         _result = null;
-          _currentFlowStep = MosaicFlowStep.result;
-        }
+        _currentFlowStep =
+            _sourceImageBytes == null ? MosaicFlowStep.image : MosaicFlowStep.result;
         _activeSection = HomeSection.mosaic;
       });
 
-      if (data.sourceImageBytes != null) await _generate();
+      if (data.sourceImageBytes != null) {
+        await _generate(catalogSnapshotOverride: hasCatalogSnapshot ? data.catalogSnapshot : null);
+      }
       if (!silent && mounted) {
+        final message = hasCatalogSnapshot
+            ? 'Projekt geladen ✅ (mit gespeichertem Katalog-Snapshot)'
+            : 'Projekt geladen ✅ (ohne Snapshot: aktueller Katalog aktiv)';
         ScaffoldMessenger.of(context).showSnackBar(
-          const SnackBar(content: Text('Projekt geladen ✅')),
+          SnackBar(content: Text(message)),
         );
       }
     } catch (_) {
@@ -879,8 +910,24 @@ class _MosaicHomePageState extends State<MosaicHomePage>
     _scheduleRegenerate();
   }
 
-  Future<void> _generate() async {
-    if (_sourceImageBytes == null || _catalog.isEmpty) return;
+  Future<void> _generate({List<MosaicPaletteSnapshotEntry>? catalogSnapshotOverride}) async {
+    if (_sourceImageBytes == null) {
+      _showMissingImageHint();
+      return;
+    }
+
+    final paletteSource = catalogSnapshotOverride != null && catalogSnapshotOverride.isNotEmpty
+        ? catalogSnapshotOverride
+            .map((entry) => <String, dynamic>{
+                  'name': entry.name,
+                  'value': entry.colorValue,
+                })
+            .toList(growable: false)
+        : _catalog
+            .map((p) => <String, dynamic>{'name': p.name, 'value': p.colorValue})
+            .toList(growable: false);
+
+    if (paletteSource.isEmpty) return;
 
     final int gridW = math.max(1, int.tryParse(_gridWidthCtrl.text) ?? 40);
     final int gridH = math.max(1, int.tryParse(_gridHeightCtrl.text) ?? 30);
@@ -899,9 +946,7 @@ class _MosaicHomePageState extends State<MosaicHomePage>
       'ditheringStrength': _ditheringStrength,
       'edgeEmphasis': _edgeEmphasis,
       'colorVariation': _colorVariation,
-      'palette': _catalog
-          .map((p) => <String, dynamic>{'name': p.name, 'value': p.colorValue})
-          .toList(growable: false),
+      'palette': paletteSource,
     };
 
     final out = await compute(_generateMosaicIsolate, payload);
@@ -952,7 +997,7 @@ class _MosaicHomePageState extends State<MosaicHomePage>
           ? FloatingActionButton.extended(
               backgroundColor: Colors.white.withValues(alpha: 0.85),
               foregroundColor: Theme.of(context).colorScheme.primary,
-              onPressed: _isGenerating ? null : _generate,
+              onPressed: _canGenerate ? _generate : null,
               icon: _isGenerating
                   ? const SizedBox(
                       width: 18,
@@ -1085,8 +1130,12 @@ class _MosaicHomePageState extends State<MosaicHomePage>
                 );
               },
               onStepContinue: () {
-                setState(() {
                 final next = _currentFlowStep.index + 1;
+                if (next > _maxAccessibleStepIndex) {
+                  _showMissingImageHint();
+                  return;
+                }
+                setState(() {
                   if (next <= MosaicFlowStep.result.index) {
                     _currentFlowStep = MosaicFlowStep.values[next];
                   }
@@ -1101,6 +1150,10 @@ class _MosaicHomePageState extends State<MosaicHomePage>
                 });
               },
               onStepTapped: (index) {
+                if (index > _maxAccessibleStepIndex) {
+                  _showMissingImageHint();
+                  return;
+                }
                 setState(() => _currentFlowStep = MosaicFlowStep.values[index]);
               },
               steps: [
@@ -1250,7 +1303,7 @@ class _MosaicHomePageState extends State<MosaicHomePage>
           children: [
             FilledButton.icon(
               key: const Key('generate-btn'),
-              onPressed: _isGenerating ? null : _generate,
+              onPressed: _canGenerate ? _generate : null,
               icon: const Icon(Icons.auto_fix_high_rounded),
               label: const Text('Generate Mosaic'),
             ),

test/widget_test.dart

@@ -1,8 +1,9 @@
+import 'package:flutter/material.dart';
 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',
+  testWidgets('stepper blocks forward navigation without image and shows hint',
       (WidgetTester tester) async {
     await tester.pumpWidget(const KorkenMosaicApp());
 
@@ -12,13 +13,23 @@ void main() {
     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();
+    await tester.pump();
 
-    expect(find.byKey(const Key('generate-btn')), findsOneWidget);
-    expect(find.text('Export JSON'), findsOneWidget);
+    // Stays on step 1 because no source image is available yet.
+    expect(find.text('Import target image'), findsOneWidget);
+    expect(find.text('Bitte zuerst ein Bild auswählen.'), findsOneWidget);
+  });
+
+  testWidgets('generate actions are disabled without image',
+      (WidgetTester tester) async {
+    await tester.pumpWidget(const KorkenMosaicApp());
+
+    final fab = tester.widget<FloatingActionButton>(
+      find.byType(FloatingActionButton),
+    );
+    expect(fab.onPressed, isNull);
+
+    // Result-step action is not reachable before image selection.
+    expect(find.byKey(const Key('generate-btn')), findsNothing);
   });
 }