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/CourseDetailsModal.vue

@@ -14,7 +14,7 @@
         <template v-else-if="course">
           <div class="course-details__hero">
             <div class="course-details__image">
-              <img v-if="course.image" :src="course.image" :alt="course.title" />
+              <img v-if="course.coverUrl" :src="course.coverUrl" :alt="course.title" />
               <SvgIcon v-else name="book" :size="32" color="#bcbcbc" />
             </div>
             <div class="course-details__grid">