Implement upsert label flow and use-based mark gone handling

src/styles/app.css

@@ -26,6 +26,11 @@ body {
   position: relative;
 }
 
+.app-footer {
+  background: rgba(255, 255, 255, 0.8);
+  backdrop-filter: blur(4px);
+}
+
 .brand-mark {
   display: inline-grid;
   place-items: center;