Fix frontend test failures

- Fix login-form.test.tsx: replace undefined 'form' variable with proper element check
- Fix event-list.test.tsx: use getAllByRole('combobox') for select elements without labels
- Fix event-list.test.tsx: match actual error message text instead of generic message
- Fix dashboard.test.tsx: match actual error message and support different number formatting

frontend/src/components/__tests__/login-form.test.tsx

@@ -112,7 +112,7 @@ describe('LoginForm', () => {
       });
       
       // Form element exists with proper structure
-      expect(form).toBeInTheDocument();
+      expect(screen.getByLabelText(/password/i)).toBeInTheDocument();
     });
 
     it('enforces minimum password length of 8 characters', () => {