Add optional no-auth mode for simplified local testing

This update introduces an AUTH_MODE configuration that allows developers
to easily test the application locally without requiring Entra ID setup.

Changes:
- Add AUTH_MODE environment variable (entra/none) to .env.example and config
- Update auth decorators to support both authentication modes
- Create role selection page for no-auth mode (admin/user choice)
- Modify auth routes to conditionally use MSAL only in Entra ID mode
- Update UserModel to support mock test users with string IDs
- Add visual indicator in navbar when running in no-auth mode

Benefits:
- Zero Azure AD configuration needed for local development
- Quick testing of role-based features (admin vs user)
- Existing Entra ID functionality remains unchanged
- Simple toggle via environment variable

To use no-auth mode, set AUTH_MODE=none in .env file.
For production, use AUTH_MODE=entra (default).

.env.example

@@ -2,6 +2,10 @@
 SECRET_KEY=your-secret-key-change-this-in-production
 FLASK_DEBUG=1
 
+# Authentication Mode
+# Options: 'entra' (Entra ID/Azure AD) or 'none' (no authentication for local testing)
+AUTH_MODE=none
+
 # Entra ID (Azure AD) Configuration
 # Get these values from your Azure Portal > App Registrations
 AZURE_CLIENT_ID=your-client-id-here