New file: lib/core/auth/auth_guard.dart
Static AuthGuard class with isGuest flag and requireLogin() helper
that shows a login prompt bottom sheet when guests try protected actions.
login_screen.dart / desktop_login_screen.dart:
Added "Continue as Guest" button below sign-up link.
Sets AuthGuard.isGuest = true, then navigates to HomeScreen.
api_client.dart:
_buildAuthBody() and GET auth check no longer throw when token is missing.
If no token (guest), request proceeds without auth — backend decides.
home_screen.dart:
Bottom nav guards: tapping Contribute (index 2) or Profile (index 3)
as guest shows login prompt instead of navigating.
auth_service.dart:
AuthGuard.setGuest(false) called on successful login AND register
so guest flag is always cleared when user authenticates.
Guest CAN: browse home, calendar, search, filter, view event details.
Guest CANNOT: contribute, view profile, book events (prompts login).
