Add comprehensive test infrastructure and Dark Mode tests

- Created test folder structure (__tests__, __mocks__)
- Added mock contexts (Theme, BiometricAuth, Auth)
- Added mock AsyncStorage
- Implemented 11 passing Dark Mode tests:
  * Rendering tests (3 tests)
  * Toggle functionality tests (2 tests)
  * Persistence tests (2 tests)
  * Theme application tests (2 tests)
  * Edge case tests (2 tests)
- Added testID props to SettingsScreen components
- All tests passing (11/11)

Test Coverage:
- Dark Mode toggle on/off
- AsyncStorage persistence
- Theme color application
- Rapid toggle handling
- Multiple toggle calls

mobile/src/__mocks__/contexts/BiometricAuthContext.tsx

@@ -0,0 +1,38 @@
+import React, { createContext, useContext, ReactNode } from 'react';
+
+// Mock Biometric Auth Context for testing
+interface BiometricAuthContextType {
+  isBiometricSupported: boolean;
+  isBiometricEnrolled: boolean;
+  isBiometricAvailable: boolean;
+  biometricType: 'fingerprint' | 'facial' | 'iris' | 'none';
+  isBiometricEnabled: boolean;
+  authenticate: () => Promise<boolean>;
+  enableBiometric: () => Promise<boolean>;
+  disableBiometric: () => Promise<void>;
+}
+
+const mockBiometricContext: BiometricAuthContextType = {
+  isBiometricSupported: true,
+  isBiometricEnrolled: true,
+  isBiometricAvailable: true,
+  biometricType: 'fingerprint',
+  isBiometricEnabled: false,
+  authenticate: jest.fn().mockResolvedValue(true),
+  enableBiometric: jest.fn().mockResolvedValue(true),
+  disableBiometric: jest.fn().mockResolvedValue(undefined),
+};
+
+const BiometricAuthContext = createContext<BiometricAuthContextType>(mockBiometricContext);
+
+export const MockBiometricAuthProvider: React.FC<{
+  children: ReactNode;
+  value?: Partial<BiometricAuthContextType>
+}> = ({ children, value = {} }) => {
+  const contextValue = { ...mockBiometricContext, ...value };
+  return <BiometricAuthContext.Provider value={contextValue}>{children}</BiometricAuthContext.Provider>;
+};
+
+export const useBiometricAuth = () => useContext(BiometricAuthContext);
+
+export default BiometricAuthContext;