fix(courses): course cover image not saved on create/edit

The admin course form sent a stray upload id (imageId/uploadId) that
didn't match any field the API accepts; the API expects cover_media_id
with the media id returned from the upload endpoint. Also switch course
displays to the coverUrl field the API actually returns.

src/features/admin/courses/components/modals/AddOfferedCourseModal.vue

@@ -154,7 +154,7 @@ const form = ref({
   templateId: '',
   title: '',
   capacity: '',
-  imageId: null,
+  coverMediaId: null,
   isActive: false,
 })
 
@@ -212,10 +212,10 @@ watch(existingCourse, (course) => {
     templateId: '',
     title: course.title || '',
     capacity: course.capacity ?? '',
-    imageId: course.imageId || null,
+    coverMediaId: course.coverMediaId || null,
     isActive: course.isActive ?? false,
   }
-  if (course.image) image.value = { url: course.image }
+  if (course.coverUrl) image.value = { url: course.coverUrl }
 })
 
 const uploadMutation = useUploadMediaMutation()
@@ -225,8 +225,8 @@ const onImageCropped = async (file) => {
     const formData = objectToFormData({ file, purpose: 'cover', context: 'course' })
     const response = await uploadMutation.mutateAsync(formData)
     const payload = response?.data ?? response
-    image.value = { url: payload?.url, uploadId: payload?.uploadId, ...payload }
-    form.value.imageId = payload?.uploadId || payload?.id
+    image.value = { url: payload?.url, ...payload }
+    form.value.coverMediaId = payload?.id
   } catch {
     /* handled globally */
   }