fix: Prevent undefined rendering in reasoning translations

Fixed React error #306 (text content mismatch) caused by undefined values
in reasoning translations.

Root cause: Translation functions could return undefined when:
- Translation key doesn't exist
- reasoning_data has unexpected structure
- parameters are missing

Changes:
- Added defaultValue to all translation calls
- Added || fallback operators to ensure strings are always returned
- Added proper null/undefined checks with empty string fallbacks
- Enhanced getInsightDescription with multiple fallback levels

This ensures all translation functions return valid strings, preventing
React hydration mismatches and rendering errors on the dashboard.

frontend/src/pages/app/analytics/ai-insights/AIInsightsPage.tsx

@@ -137,13 +137,18 @@ const AIInsightsPage: React.FC = () => {
       }
 
       try {
-        return t(`${category}.${insight.reasoning_data.type}`, insight.reasoning_data.parameters);
+        const params = {
+          ...insight.reasoning_data.parameters,
+          defaultValue: insight.description || `${category}: ${insight.reasoning_data.type}`
+        };
+        const translated = t(`${category}.${insight.reasoning_data.type}`, params);
+        return translated || insight.description || `AI Insight: ${insight.reasoning_data.type}`;
       } catch (e) {
         // Fall back to description if translation key not found
-        return insight.description;
+        return insight.description || 'AI Insight';
       }
     }
-    return insight.description;
+    return insight.description || 'AI Insight';
   };
 
   return (