feat(ui,api,telegram): shared conversation panel for obsidian and /admin/panel, bind materializes a window, inbox survives link previews

ui/src/lib/api/client.ts

@@ -245,14 +245,15 @@ export class ApiClient {
     return this.post(`/api/conversations/${id}/close`);
   }
 
+  // Without ``externalId`` the frontend opens a fresh window (a new topic).
   bind(
     id: string,
     frontend: string,
-    externalId: string,
+    externalId: string | null,
     visible: boolean
   ): Promise<ConversationInfo> {
     return this.post(`/api/conversations/${id}/bind`, {
-      external_id: externalId,
+      external_id: externalId ?? undefined,
       frontend,
       visible,
     });