Task T10 evidence: no Namenseinblender macro configured

Test coverage:
- `build returns null when namenseinblender macro is not configured`
- Verifies `NameTagSlideBuilder::build('Max Mustermann', 'Moderation')` returns `null` when `namenseinblender_macro_name` is missing.

Targeted verification:
- `ddev exec php artisan test tests/Feature/NameTagSlideBuilderTest.php`
- Result: 3 passed (4 assertions).

Full verification:
- `ddev exec php artisan test`
- Result: 544 passed (2703 assertions).