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/public/assets/css/admin.css

@@ -147,6 +147,43 @@
     padding-right: 1.2rem;
 }
 
+/* Required-field indicator */
+.admin-form
+    input:not([type="checkbox"]):not([type="radio"]):not([type="file"]):not(
+        [type="hidden"]
+    ):not([type="submit"]):required,
+.admin-form select:required,
+.admin-form textarea:required {
+    border-bottom-style: dashed;
+}
+
+.admin-form
+    div:has(
+        input:required:not([type="checkbox"]):not([type="radio"]):not([type="file"]):not([type="hidden"])) > label,
+.admin-form div:has(select:required) > label,
+.admin-form div:has(textarea:required) > label {
+    font-weight: 600;
+}
+
+/* Required-field indicator (student form / generic labels) */
+label:has(+ input:required:not([type="hidden"]))::after,
+label:has(+ select:required)::after,
+label:has(+ textarea:required)::after,
+label:has(+ div > input:required)::after {
+    content: " *";
+    color: var(--error, #c00);
+}
+
+/* Visually-hidden "required fields marked with *" note */
+.required-note {
+    font-size: var(--step--2);
+    color: var(--text-secondary);
+    margin-bottom: var(--space-xs);
+}
+.required-note .asterisk {
+    color: var(--error, #c00);
+}
+
 /* File inputs */
 .admin-file-input {
     display: flex;