Fix unresolved Activity component warning in ActivityFeed.vue

The lucide icon was imported as "Activity as ActivityIcon" but the empty-
state template still referenced <Activity>, which was never registered
under that name — triggering a Vue "failed to resolve component" warning
whenever a project's activity feed was empty.

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

frontend/src/components/activity/ActivityFeed.vue

@@ -34,7 +34,7 @@
         </div>
 
         <div v-else-if="activities.length === 0" class="text-center py-8 text-muted-foreground">
-          <Activity class="h-12 w-12 mx-auto mb-2 opacity-50" />
+          <ActivityIcon class="h-12 w-12 mx-auto mb-2 opacity-50" />
           <p>No activity yet</p>
         </div>