feat(ai): enhance AI trace warnings and reason codes system

- Added structured warning system with `AdminAiWarning` type in backend and Flutter
- Implemented detailed reason descriptors with `FlyerReasonDescriptor` for parse and match operations
- Added `legacyWarnings` field to maintain backward compatibility
- Enhanced AI trace service to collect and format warnings with item-level context
- Updated flyer import services to include detailed reason descriptions in responses
- Added Swedish diacritic preservation for cheese variants (Prästost, Herrgårdsost, Grevéost)
- Implemented UTF-8 content validation for AI responses
- Added new reason code definitions in `reason-codes.ts`
- Updated Flutter UI to display structured warnings with severity indicators
- Added error report generation and copy functionality in admin panel
- Added comprehensive test coverage for new warning system and cheese normalization

BREAKING CHANGE: AI trace warnings are now structured objects instead of simple strings

flutter/test/features/admin/presentation/admin_ai_panel_test.dart

@@ -86,7 +86,28 @@ void main() {
     durationMs: 1880,
     retryCount: 1,
     chunkCount: 3,
-    warnings: const ['parse:low_confidence', 'match:no_match'],
+    warnings: const [
+      AdminAiWarning(
+        code: 'low_confidence',
+        kind: 'parse',
+        title: 'Låg parsningskvalitet',
+        message: 'Modellens säkerhet är låg, granska raden manuellt.',
+        severity: 'warning',
+        location: 'Steg: AI-parser',
+        itemIndex: 5,
+      ),
+      AdminAiWarning(
+        code: 'no_match',
+        kind: 'match',
+        title: 'Ingen produktmatchning',
+        message:
+            'Vi kunde inte hitta någon befintlig produkt som matchar texten på flyern.',
+        severity: 'warning',
+        location: 'Steg: matchning mot dina produkter',
+        itemIndex: 7,
+      ),
+    ],
+    legacyWarnings: const ['parse:low_confidence', 'match:no_match'],
     error: null,
     prompt: 'Prompttext exempel',
     rawOutput: veryLargeOutput,
@@ -198,8 +219,8 @@ void main() {
 
       expect(find.text('Sammanfattning'), findsOneWidget);
       expect(find.text('Varningar (2)'), findsOneWidget);
-      expect(find.text('parse:low_confidence'), findsOneWidget);
-      expect(find.text('match:no_match'), findsOneWidget);
+      expect(find.text('Låg parsningskvalitet'), findsOneWidget);
+      expect(find.text('Ingen produktmatchning'), findsOneWidget);
       final detailScroll = find.byType(Scrollable).last;
       await tester.scrollUntilVisible(
         find.text('Model Output'),
@@ -219,9 +240,11 @@ void main() {
       final copyPrompt = find.byTooltip('Kopiera');
       final copyOutput = find.byTooltip('Kopiera JSON');
       final copyWarnings = find.byTooltip('Kopiera alla varningar');
+      final copyErrorReport = find.text('Kopiera felrapport');
       expect(copyPrompt, findsOneWidget);
       expect(copyOutput, findsOneWidget);
       expect(copyWarnings, findsOneWidget);
+      expect(copyErrorReport, findsOneWidget);
 
       await tester.tap(copyPrompt);
       await tester.pumpAndSettle();
@@ -235,6 +258,10 @@ void main() {
       await tester.pumpAndSettle();
       expect(tester.takeException(), isNull);
 
+      await tester.tap(copyErrorReport);
+      await tester.pumpAndSettle();
+      expect(tester.takeException(), isNull);
+
       addTearDown(() => tester.binding.setSurfaceSize(null));
     });
   });