[Frontend] Implement Mock Authentication Flow for Expo Go Compatibility

[MohamadNazik]

Description:
The mobile application currently relies on react-native-app-auth, a native module not supported by Expo Go. This prevents UI development and testing on physical devices without full native builds.

Proposed Solution:
Implement a mock authentication toggle (EXPO_PUBLIC_MOCK_AUTH=true) to bypass native module calls.

Acceptance Criteria:

• The app can be launched on a physical device using the Expo Go app without crashing.
• When EXPO_PUBLIC_MOCK_AUTH=true, clicking the "Sign In" button skips the authorize() call and immediately updates the app state to isAuthenticated: true.
• A hardcoded mock user profile and token are provided to the rest of the application.
• Authenticated API calls to the local backend use the mock token instead of failing.
• Native-only features (Notifications, Google Sign-In) are gracefully caught to prevent "null reference" errors in Expo Go.