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__/@react-native-async-storage/async-storage.ts

@@ -0,0 +1,42 @@
+// Mock AsyncStorage for testing
+const storage: { [key: string]: string } = {};
+
+export default {
+  setItem: jest.fn((key: string, value: string) => {
+    storage[key] = value;
+    return Promise.resolve();
+  }),
+  getItem: jest.fn((key: string) => {
+    return Promise.resolve(storage[key] || null);
+  }),
+  removeItem: jest.fn((key: string) => {
+    delete storage[key];
+    return Promise.resolve();
+  }),
+  clear: jest.fn(() => {
+    Object.keys(storage).forEach(key => delete storage[key]);
+    return Promise.resolve();
+  }),
+  getAllKeys: jest.fn(() => {
+    return Promise.resolve(Object.keys(storage));
+  }),
+  multiGet: jest.fn((keys: string[]) => {
+    return Promise.resolve(
+      keys.map(key => [key, storage[key] || null])
+    );
+  }),
+  multiSet: jest.fn((keyValuePairs: [string, string][]) => {
+    keyValuePairs.forEach(([key, value]) => {
+      storage[key] = value;
+    });
+    return Promise.resolve();
+  }),
+  multiRemove: jest.fn((keys: string[]) => {
+    keys.forEach(key => delete storage[key]);
+    return Promise.resolve();
+  }),
+  _clear: () => {
+    Object.keys(storage).forEach(key => delete storage[key]);
+  },
+  _getStorage: () => storage,
+};