fix(frontend): wait for availability check before submit in test

The test was clicking submit before isAvailable was set to true,
causing intermittent failures in CI. Now waits for "Ce nom est
disponible" message to appear before clicking Dupliquer.

Author: pierrick-fonquerne

src/frontend/src/components/character/DuplicateModal.test.tsx

@@ -154,8 +154,9 @@ describe('DuplicateModal', () => {
       await user.clear(input);
       await user.type(input, '  NewCharacter  ');
 
+      // Wait for availability check to complete (not just be called)
       await waitFor(() => {
-        expect(characterService.checkNameAvailability).toHaveBeenCalled();
+        expect(screen.getByText('Ce nom est disponible')).toBeInTheDocument();
       });
 
       await user.click(screen.getByText('Dupliquer'));