Prevent duplicate post submissions

static/app.css

@@ -41,6 +41,31 @@ body {
   background-size: 100% 100%, 48px 48px;
 }
 
+.submit-progress {
+  position: fixed;
+  inset: 0 0 auto;
+  z-index: 100;
+  height: 3px;
+  overflow: hidden;
+  pointer-events: none;
+}
+
+.submit-progress[hidden] { display: none; }
+
+.submit-progress-bar {
+  display: block;
+  width: 35%;
+  height: 100%;
+  background: var(--signal);
+  box-shadow: 0 0 12px var(--signal);
+  animation: submit-progress 900ms ease-in-out infinite;
+}
+
+@keyframes submit-progress {
+  from { transform: translateX(-100%); }
+  to { transform: translateX(300%); }
+}
+
 img, svg { display: block; }
 
 a {
@@ -253,6 +278,12 @@ a {
 
 .btn-primary:hover { filter: brightness(1.08); }
 
+.btn:disabled {
+  cursor: wait;
+  opacity: 0.65;
+  filter: none;
+}
+
 .btn-ghost {
   background: transparent;
   color: var(--ink);
@@ -763,6 +794,10 @@ input:focus, textarea:focus, .btn:focus-visible, .chip:focus-visible, .vote-btn:
 
 @media (prefers-reduced-motion: reduce) {
   .btn-primary:hover { filter: none; }
+  .submit-progress-bar {
+    width: 100%;
+    animation: none;
+  }
 }
 
 .admin-users { margin-top: 20px; overflow-x: auto; }