feat(input): drag-drop + paste-as-attachment for long text

apps/web/src/components/DropOverlay.tsx

@@ -0,0 +1,18 @@
+interface Props {
+  visible: boolean;
+}
+
+// Visual cue layered over the ChatInput while a drag is in progress.
+// Pointer-events: none so the underlying drop handler still receives the
+// drop event. Renders nothing when not visible (cheap and out of layout).
+export function DropOverlay({ visible }: Props) {
+  if (!visible) return null;
+  return (
+    <div
+      className="absolute inset-0 z-10 pointer-events-none flex items-center justify-center rounded border-2 border-dashed border-primary bg-background/85"
+      aria-hidden="true"
+    >
+      <div className="text-sm font-medium text-primary">Drop to attach</div>
+    </div>
+  );
+}