feat: Wire up brand kit — favicon, logos, and hero mark across all layouts

Copies brand assets to frontend/public (favicon.png, logo.png, logo-hero.png).
Updates index.html, LoginView, RegisterView, DashboardLayout sidebar, and
PublicLayout footer with proper branding.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/layout/PublicLayout.vue

@@ -7,7 +7,10 @@ import { RouterView } from 'vue-router'
     <RouterView />
 
     <footer class="py-6 text-center text-neutral-600 text-sm border-t border-neutral-800">
-      Powered by <span class="text-red-500 font-semibold">Corrosion</span>
+      <div class="flex items-center justify-center gap-2">
+        <img src="/logo.png" alt="Corrosion" class="h-4 w-4 opacity-60" />
+        <span>Powered by <span class="text-red-500 font-semibold">Corrosion</span></span>
+      </div>
     </footer>
   </div>
 </template>