fix: mark languages as required, add required-field visual indicators on both forms

- checkbox-list.php: support $required prop → adds required + aria-required on fieldset
- add.php: languages checkbox now marked required (matches server-side validation)
- partage/index.php: same for student form
- admin.css: dashed border on required inputs, bold labels, red asterisk via :has(), "Champs obligatoires" note
- Both forms now show "* Champs obligatoires" note at top

Server-side required fields = titre, auteurice, synopsis, année, orientation, ap, finality, languages (≥1), access_type_id, confirmation_email. All now have required attribute + visual asterisk.

app/templates/partials/form/checkbox-list.php

@@ -12,13 +12,15 @@
  *   string  $label     — group label text
  *   array   $options   — each element must have 'id' and 'name' keys
  *   array   $checked   — array of 'id' values that are currently checked
+ *   bool    $required  — whether at least one checkbox must be checked; default false
  */
 
-$checked = $checked ?? [];
+$checked  = $checked  ?? [];
+$required = $required ?? false;
 ?>
 <div>
     <span class="admin-row-label"><?= htmlspecialchars($label) ?></span>
-    <fieldset class="admin-checkbox-group">
+    <fieldset class="admin-checkbox-group"<?= $required ? ' required aria-required="true"' : '' ?>>
         <legend class="sr-only"><?= htmlspecialchars($label) ?></legend>
         <ul>
             <?php foreach ($options as $opt): ?>