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/ThemeContext.tsx

@@ -0,0 +1,43 @@
+import React, { createContext, useContext, ReactNode } from 'react';
+
+// Mock colors instead of importing from shared
+const LightColors = {
+  background: '#F5F5F5',
+  surface: '#FFFFFF',
+  text: '#000000',
+  textSecondary: '#666666',
+  border: '#E0E0E0',
+};
+
+// Mock Theme Context for testing
+interface ThemeContextType {
+  isDarkMode: boolean;
+  toggleDarkMode: () => Promise<void>;
+  colors: typeof LightColors;
+  fontSize: 'small' | 'medium' | 'large';
+  setFontSize: (size: 'small' | 'medium' | 'large') => Promise<void>;
+  fontScale: number;
+}
+
+const mockThemeContext: ThemeContextType = {
+  isDarkMode: false,
+  toggleDarkMode: jest.fn().mockResolvedValue(undefined),
+  colors: LightColors,
+  fontSize: 'medium',
+  setFontSize: jest.fn().mockResolvedValue(undefined),
+  fontScale: 1,
+};
+
+const ThemeContext = createContext<ThemeContextType>(mockThemeContext);
+
+export const MockThemeProvider: React.FC<{ children: ReactNode; value?: Partial<ThemeContextType> }> = ({
+  children,
+  value = {}
+}) => {
+  const contextValue = { ...mockThemeContext, ...value };
+  return <ThemeContext.Provider value={contextValue}>{children}</ThemeContext.Provider>;
+};
+
+export const useTheme = () => useContext(ThemeContext);
+
+export default ThemeContext;