fix(background-store): rename store id to avoid collision with stage-layouts

packages/stage-ui/src/stores/background.ts

@@ -35,7 +35,16 @@ const BUILTIN_BACKGROUNDS = [
   },
 ]
 
-export const useBackgroundStore = defineStore('background', () => {
+// NOTICE:
+// id is `background-entries` (not `background`) to avoid colliding with
+// stage-layouts' `defineStore('background', ...)` — Pinia uses the string id
+// as a global singleton key, so two stores with the same id resolve to
+// whichever was registered first at runtime (TS cannot detect this since the
+// shape is inferred per-module).
+// Source: packages/stage-layouts/src/stores/background.ts.
+// Removal condition: when this store is merged with stage-layouts' store, or
+// the string id collision is enforced at the type level.
+export const useBackgroundStore = defineStore('background-entries', () => {
   const STORAGE_PREFIX = 'bg-'
 
   const entries = ref<Map<string, BackgroundEntry>>(new Map())