Add fetch timeout/retry, friendly error messages, and VAT rate constant

- fetchWithTimeout (10s) + fetchWithRetry (3 attempts, exponential backoff)
- DataError class with type classification (config/network/auth/timeout/unknown)
- User-friendly error messages in EN/AR instead of raw error strings
- Extract VAT_RATE constant (was hardcoded 1.15)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/App.tsx

@@ -5,7 +5,8 @@ import Comparison from './components/Comparison';
 import Slides from './components/Slides';
 import { fetchData, getCacheStatus, refreshData } from './services/dataService';
 import { useLanguage } from './contexts/LanguageContext';
-import type { MuseumRecord, CacheStatus } from './types';
+import type { MuseumRecord, CacheStatus, DataErrorType } from './types';
+import { DataError } from './types';
 import './App.css';
 
 interface NavLinkProps {
@@ -35,7 +36,7 @@ function App() {
   const [data, setData] = useState<MuseumRecord[]>([]);
   const [loading, setLoading] = useState<boolean>(true);
   const [refreshing, setRefreshing] = useState<boolean>(false);
-  const [error, setError] = useState<string | null>(null);
+  const [error, setError] = useState<{ message: string; type: DataErrorType } | null>(null);
   const [isOffline, setIsOffline] = useState<boolean>(false);
   const [cacheInfo, setCacheInfo] = useState<CacheStatus | null>(null);
   const [showDataLabels, setShowDataLabels] = useState<boolean>(false);
@@ -62,7 +63,8 @@ function App() {
       const status = getCacheStatus();
       setCacheInfo(status);
     } catch (err) {
-      setError((err as Error).message);
+      const type = err instanceof DataError ? err.type : 'unknown';
+      setError({ message: (err as Error).message, type });
       console.error(err);
     } finally {
       setLoading(false);
@@ -92,8 +94,10 @@ function App() {
     return (
       <div className="error-container" dir={dir}>
         <h2>{t('app.error')}</h2>
-        <p style={{maxWidth: '400px', textAlign: 'center', color: '#64748b'}}>{error}</p>
-        <button onClick={() => window.location.reload()}>{t('app.retry')}</button>
+        <p style={{maxWidth: '400px', textAlign: 'center', color: '#64748b'}}>
+          {t(`errors.${error.type}`)}
+        </p>
+        <button onClick={() => loadData()}>{t('app.retry')}</button>
       </div>
     );
   }