refactor(desktop): trim backdrop store to match tool-view style

apps/desktop/src/store/backdrop.ts

@@ -1,21 +1,14 @@
-/**
- * Chat backdrop image (the faint statue behind the transcript). One boolean,
- * on by default. Purely presentational renderer state — the `Backdrop`
- * component just skips rendering when it's off.
- */
-
 import { atom } from 'nanostores'
 
 import { persistBoolean, storedBoolean } from '@/lib/storage'
 
 const KEY = 'hermes.desktop.backdrop.v1'
 
-export const $backdrop = atom<boolean>(typeof window === 'undefined' ? true : storedBoolean(KEY, true))
+/** Whether the faint statue image renders behind the chat transcript. */
+export const $backdrop = atom(storedBoolean(KEY, true))
 
-export function setBackdrop(on: boolean): void {
+$backdrop.subscribe(on => persistBoolean(KEY, on))
+
+export function setBackdrop(on: boolean) {
   $backdrop.set(on)
 }
-
-if (typeof window !== 'undefined') {
-  $backdrop.subscribe(on => persistBoolean(KEY, on))
-}