feat(frontend): visibility note for standalone assessment-type activities

CourseView.jsx already special-cased 'capstone' activities with a pointer to
the Capstone tab (since only capstone-type activities existed before this
phase). Phase 6C's new rootnode-security-assessment introduces the first
non-capstone 'assessment'-type module activity — without a similar note, a
student would see it in the course tree with no way to discover it needs to
be started from the Assessments tab. Adds the same pattern for 'assessment'
type, naming the assessment by its own title.

No other frontend change was needed: CurriculumView.jsx (instructor view)
already renders activity_type labels, scenario_id/assessment_definition_id,
is_required, instructor_notes availability, and READY/PARTIAL/PLANNED status
pills generically — Phase 6C's new module/activity shapes render correctly
under Phase 6B's existing generic logic, verified by inspection.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

frontend/src/platform/CourseView.jsx

@@ -210,6 +210,9 @@ function ActivityDetail({ activity, onLaunchLab, onChange }) {
         {activity.activity_type === 'capstone' && (
           <p className={styles.muted}>Continue via the Capstone tab to start or resume this assessment.</p>
         )}
+        {activity.activity_type === 'assessment' && !activity.complete && (
+          <p className={styles.muted}>Continue via the Assessments tab to start or resume &ldquo;{activity.title}&rdquo;.</p>
+        )}
       </div>
     </div>
   )