refactor(webapp): drop the unused isDemoChatId helper

Never exported from the demo barrel and never called on any branch in the stack.

Author: kathiekiwi

apps/webapp/app/components/dashboard-agent/demo/ids.ts

@@ -10,10 +10,6 @@ export function demoId(rest: string): string {
   return `${DEMO_ID_PREFIX}${rest}`;
 }
 
-export function isDemoChatId(id: string | null | undefined): boolean {
-  return typeof id === "string" && id.startsWith(DEMO_ID_PREFIX);
-}
-
 export const DEMO_PROJECT_REF = "proj_demo00000000000000";
 export const DEMO_ENVIRONMENT_ID = "env_demo00000000000000";