feat: add drag-and-drop file upload to Cloud folders

Drag files over the native file browser area to see a drop zone overlay
with dashed orange border. Dropping files triggers the existing upload
handler. Uses debounced dragleave to prevent flicker between children.

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

neode-ui/src/style.css

@@ -1220,3 +1220,31 @@ html:has(body.video-background-active)::before {
   border-radius: 2px;
   transition: width 0.3s linear;
 }
+
+/* ── Cloud Drag-and-Drop Overlay ──── */
+
+.cloud-drop-overlay {
+  position: absolute;
+  inset: 0;
+  z-index: 30;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: rgba(0, 0, 0, 0.6);
+  backdrop-filter: blur(8px);
+  -webkit-backdrop-filter: blur(8px);
+  border-radius: 0.75rem;
+  border: 2px dashed rgba(251, 146, 60, 0.6);
+  animation: drop-overlay-in 0.2s ease-out;
+}
+.cloud-drop-overlay-inner {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  text-align: center;
+  padding: 2rem;
+}
+@keyframes drop-overlay-in {
+  from { opacity: 0; }
+  to { opacity: 1; }
+}