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

backend/src/flyer-import/services/ai-flyer-parser.service.ts

@@ -214,10 +214,11 @@ Regler:
 6) Om en rubrik/lista innehaller flera kommaseparerade namn och efterfoljande rad/rader innehaller gemensam brand, vikt, pris eller kampanjvillkor: expandera till separata objekt (en per namn) och arv all gemensam metadata.
 7) Tillämpa samma split-regel generellt for liknande tillbud (inte bara ost), nar listan tydligt representerar produktvarianter/smaker/sorter.
 8) Splitta INTE om listan snarare ar ingredienser, avdelningar, eller otydlig marknadsforing utan tydlig produktvariant.
-9) Specialregel ost: namn som PRAST/HERRGARD/GREVE ska normaliseras till Prastost/Herrgardsost/Greveost.
+9) Specialregel ost: namn som PRAST/HERRGARD/GREVE ska normaliseras till Prästost/Herrgårdsost/Grevéost.
 10) Om texten innehaller "ARLA KO" ska brand vara exakt "Arla Ko".
 11) For ovan ostsorter ska category vara "Hardost".
-12) Returnera aldrig extra nycklar, text, markdown eller forklaringar utanfor JSON-arrayen.
+12) Behåll svenska diakritiska tecken (ä, å, ö, é) i produktnamn. Returnera "Prästost", "Herrgårdsost", "Grevéost" - inte ASCII-versioner.
+13) Returnera aldrig extra nycklar, text, markdown eller forklaringar utanfor JSON-arrayen.
 
 Exempel bundle utdata:
 [
@@ -258,7 +259,7 @@ Input-idé: "PRAST, HERRGARD, GREVE" + "ARLA KO" + gemensam vikt/pris.
 Output-idé:
 [
   {
-    "name": "Prastost",
+    "name": "Prästost",
     "brand": "Arla Ko",
     "category": "Hardost",
     "isBundle": false,
@@ -271,7 +272,7 @@ Output-idé:
     "offer": ["Max 3 forp/hushall"]
   },
   {
-    "name": "Herrgardsost",
+    "name": "Herrgårdsost",
     "brand": "Arla Ko",
     "category": "Hardost",
     "isBundle": false,
@@ -358,7 +359,7 @@ ${truncatedText}`;
   private normalizeName(name: string): string {
     return name
       .toLowerCase()
-      .replace(/[^a-zåäö0-9\s]/g, '')
+      .replace(/[^a-zåäöé0-9\s]/g, '')
       .replace(/\s+/g, ' ')
       .trim();
   }
@@ -427,7 +428,7 @@ ${truncatedText}`;
           'Mistral-anrop timeout',
         );
 
-        const content = response.choices?.[0]?.message?.content;
+        const content = this.ensureUtf8Content(response.choices?.[0]?.message?.content);
         if (!content) {
           throw new BadRequestException('Tomt svar från AI-modellen.');
         }
@@ -531,6 +532,40 @@ ${truncatedText}`;
     return hasCampaignMarkers ? normalized : '';
   }
 
+  private ensureUtf8Content(content: unknown): string {
+    const asString = this.flattenContent(content);
+    if (!asString) return '';
+
+    const utf8 = Buffer.from(asString, 'utf8').toString('utf8');
+    if (this.debugEnabled && (asString.includes('\uFFFD') || utf8.includes('\uFFFD'))) {
+      const hex = Buffer.from(asString, 'utf8').toString('hex').slice(0, 256);
+      this.logger.debug(`Potential encoding issue in AI response (hex preview): ${hex}`);
+    }
+    return utf8;
+  }
+
+  private flattenContent(content: unknown): string {
+    if (typeof content === 'string') {
+      return content;
+    }
+    if (Array.isArray(content)) {
+      return content
+        .map((part) => {
+          if (typeof part === 'string') return part;
+          if (part && typeof part === 'object' && 'text' in part) {
+            const text = (part as { text?: unknown }).text;
+            return typeof text === 'string' ? text : '';
+          }
+          return '';
+        })
+        .join('');
+    }
+    if (content == null) {
+      return '';
+    }
+    return String(content);
+  }
+
   private readPositiveIntEnv(key: string, fallback: number): number {
     const raw = process.env[key];
     if (!raw) return fallback;