fix(exam): lock the exam form to CHOICE when administration is Online

Matches the new backend rule (online_exam_requires_choice_form) — picking
Online in the exam form now auto-sets Form to Choice and disables that
field with an explanatory hint, instead of letting the user pick Essay
and only finding out at save time.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/backoffice/src/app/features/exam/pages/ExamPage/index.tsx

@@ -254,6 +254,12 @@ function ExamForm({
                   onChange={setForm}
                   size="sm"
                   required
+                  disabled={adminMethod === "ONLINE"}
+                  description={
+                    adminMethod === "ONLINE"
+                      ? t("exam.form.onlineChoiceOnlyHint")
+                      : undefined
+                  }
                 />
                 <Select
                   label={t("exam.detail.administration")}
@@ -263,7 +269,14 @@ function ExamForm({
                     { value: "ONLINE", label: t("exam.form.online") },
                   ]}
                   value={adminMethod}
-                  onChange={setAdminMethod}
+                  onChange={(value) => {
+                    setAdminMethod(value);
+                    // Online exams are graded automatically, and that only
+                    // has an answer model for CHOICE — matches the backend
+                    // rule (online_exam_requires_choice_form), not just a
+                    // UI nicety.
+                    if (value === "ONLINE") setForm("CHOICE");
+                  }}
                   size="sm"
                   required
                 />