🎨(frontend) mark non-reassigned private attributes as readonly

Add readonly modifier to private class attributes that are never reassigned
after initialization. Improves code safety and clearly communicates immutable
intent to other developers.

src/frontend/src/features/sdk/utils/CallbackIdHandler.ts

@@ -1,5 +1,5 @@
 export class CallbackIdHandler {
-  private storageKey = 'popup_callback_id'
+  private readonly storageKey = 'popup_callback_id'
 
   private generateId(): string {
     return (