feat(webui): preview dragged session mentions

webui/src/lib/session-drag.ts

@@ -1,18 +1,25 @@
 export const SESSION_DRAG_TYPE = "application/x-nanobot-session-key";
 
+let activeSessionKey: string | null = null;
+
 export function hasDraggedSession(dataTransfer: DataTransfer): boolean {
   return Array.from(dataTransfer.types).includes(SESSION_DRAG_TYPE);
 }
 
 export function readDraggedSession(dataTransfer: DataTransfer): string | null {
   const sessionKey = dataTransfer.getData(SESSION_DRAG_TYPE).trim();
-  return sessionKey || null;
+  return sessionKey || activeSessionKey;
+}
+
+export function clearDraggedSession(): void {
+  activeSessionKey = null;
 }
 
 export function writeDraggedSession(
   dataTransfer: DataTransfer,
   sessionKey: string,
 ): void {
+  activeSessionKey = sessionKey;
   dataTransfer.effectAllowed = "copyMove";
   dataTransfer.setData(SESSION_DRAG_TYPE, sessionKey);
 }