Fix all shadow deprecation warnings across entire mobile app

- Replaced shadowColor/shadowOffset/shadowOpacity/shadowRadius with boxShadow
- Fixed 28 files (21 screens + 7 components)
- Preserved elevation for Android compatibility
- All React Native Web deprecation warnings resolved

Files fixed:
- All screen components
- All reusable components
- Navigation components
- Modal components

mobile/src/components/BalanceCard.tsx

@@ -66,10 +66,7 @@ const styles = StyleSheet.create({
     padding: 16,
     borderRadius: 12,
     marginBottom: 12,
-    shadowColor: '#000',
-    shadowOffset: { width: 0, height: 2 },
-    shadowOpacity: 0.1,
-    shadowRadius: 4,
+    boxShadow: '0px 2px 4px rgba(0, 0, 0, 0.1)',
     elevation: 3,
   },
   row: {