fix(mobile): critical security and error handling improvements

🔐 SECURITY FIXES:
- Fixed CRITICAL seed storage vulnerability
  * Changed from AsyncStorage to SecureStore for wallet seeds
  * Seeds now encrypted in hardware-backed secure storage
  * Affects: PolkadotContext.tsx (lines 166, 189)

🛡️ ERROR HANDLING:
- Added global ErrorBoundary component
  * Catches unhandled React errors
  * Shows user-friendly error UI
  * Integrated into App.tsx provider hierarchy
  * Files: ErrorBoundary.tsx (new), App.tsx, components/index.ts

🧹 PRODUCTION READINESS:
- Protected all 47 console statements with __DEV__ checks
  * console.log: 12 statements
  * console.error: 32 statements
  * console.warn: 1 statement
  * Files affected: 16 files across contexts, screens, i18n
  * Production builds will strip these out

📦 PROVIDER HIERARCHY:
- Added BiometricAuthProvider to App.tsx
- Updated provider order:
  ErrorBoundary → Polkadot → Language → BiometricAuth → Navigator

Files modified: 18
New files: 1 (ErrorBoundary.tsx)

This commit resolves 3 P0 critical issues from production readiness audit.

mobile/App.tsx

@@ -2,8 +2,10 @@ import React, { useEffect, useState } from 'react';
 import { View, ActivityIndicator, StyleSheet } from 'react-native';
 import { StatusBar } from 'expo-status-bar';
 import { initializeI18n } from './src/i18n';
+import { ErrorBoundary } from './src/components/ErrorBoundary';
 import { LanguageProvider } from './src/contexts/LanguageContext';
 import { PolkadotProvider } from './src/contexts/PolkadotContext';
+import { BiometricAuthProvider } from './src/contexts/BiometricAuthContext';
 import AppNavigator from './src/navigation/AppNavigator';
 import { KurdistanColors } from './src/theme/colors';
 
@@ -35,12 +37,16 @@ export default function App() {
   }
 
   return (
-    <PolkadotProvider>
-      <LanguageProvider>
-        <StatusBar style="auto" />
-        <AppNavigator />
-      </LanguageProvider>
-    </PolkadotProvider>
+    <ErrorBoundary>
+      <PolkadotProvider>
+        <LanguageProvider>
+          <BiometricAuthProvider>
+            <StatusBar style="auto" />
+            <AppNavigator />
+          </BiometricAuthProvider>
+        </LanguageProvider>
+      </PolkadotProvider>
+    </ErrorBoundary>
   );
 }