fix: auth token handling, add tests

- Read bearer token from set-auth-token header
- Add mounted checks to prevent setState after dispose
- Add mocktail for testing
- Add widget tests for login, clients, events screens
- Add unit tests for auth provider, API client
- 110 tests passing

lib/features/auth/presentation/register_screen.dart

@@ -46,9 +46,11 @@ class _RegisterScreenState extends ConsumerState<RegisterScreen> {
         context.go('/');
       }
     } catch (e) {
-      setState(() {
-        _error = 'Registration failed. Please try again.';
-      });
+      if (mounted) {
+        setState(() {
+          _error = 'Registration failed. Please try again.';
+        });
+      }
     } finally {
       if (mounted) {
         setState(() {