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/screens/SignUpScreen.tsx

@@ -204,10 +204,7 @@ const styles = StyleSheet.create({
     justifyContent: 'center',
     alignItems: 'center',
     marginBottom: 16,
-    shadowColor: '#000',
-    shadowOffset: { width: 0, height: 4 },
-    shadowOpacity: 0.3,
-    shadowRadius: 8,
+    boxShadow: '0px 4px 8px rgba(0, 0, 0, 0.3)',
     elevation: 8,
   },
   logoText: {
@@ -230,10 +227,7 @@ const styles = StyleSheet.create({
     backgroundColor: KurdistanColors.spi,
     borderRadius: 20,
     padding: 24,
-    shadowColor: '#000',
-    shadowOffset: { width: 0, height: 4 },
-    shadowOpacity: 0.2,
-    shadowRadius: 8,
+    boxShadow: '0px 4px 8px rgba(0, 0, 0, 0.2)',
     elevation: 8,
   },
   inputGroup: {
@@ -259,10 +253,7 @@ const styles = StyleSheet.create({
     padding: 16,
     alignItems: 'center',
     marginTop: 8,
-    shadowColor: KurdistanColors.sor,
-    shadowOffset: { width: 0, height: 4 },
-    shadowOpacity: 0.3,
-    shadowRadius: 6,
+    boxShadow: '0px 4px 6px rgba(255, 0, 0, 0.3)',
     elevation: 6,
   },
   signUpButtonText: {