fix: address PR review feedback on help texts and translations

- Replace 'speelronde' with 'spel' in index and season_add help
- Season settings help: remove incorrect claim name is editable, describe
  actual settings (Show Numbers, Confirm Answers)
- quiz_add help: rename 'XLSX-bestand' to 'Excel-bestand', add explanation
  of WAAR/ONWAAR (Dutch Excel) vs TRUE/FALSE (English Excel) for marking
  the correct answer
- quiz_answer_mapping help: remove em-dashes
- quiz_candidates help: clarify that multiple devices and mixed setups
  (multiple laptops, phones, or a mix) are supported
- quiz_question_bank_form help: change 'thema' to 'type vraag' for labels
- Rename 'Add from XLSX' to 'Import' in translation and template

templates/backoffice/season/tab_tests.html.twig

@@ -2,7 +2,7 @@
     <div class="col-md-6 col-12">
         <div class="d-flex align-items-center gap-2 mb-3">
             <a class="btn btn-sm btn-outline-primary"
-               href="{{ path('tvdt_backoffice_quiz_add', {seasonCode: season.seasonCode}) }}">{{ 'Add from XLSX'|trans }}</a>
+               href="{{ path('tvdt_backoffice_quiz_add', {seasonCode: season.seasonCode}) }}">{{ 'Import'|trans }}</a>
             <a class="btn btn-sm btn-outline-secondary"
                href="{{ path('tvdt_backoffice_quiz_add_blank', {seasonCode: season.seasonCode}) }}">{{ 'Add blank'|trans }}</a>
         </div>