✨(frontend) create STORAGE_KEYS constant

Add STORAGE_KEYS object to centralize localStorage keys,
ensuring no key overlaps by maintaining a single source
of truth for key declarations across the app.

Might be premature, as only the notification store will be persisted.

src/frontend/src/utils/storageKeys.tsx

@@ -0,0 +1,6 @@
+/**
+ * Object containing all localStorage keys used across the app
+ */
+export const STORAGE_KEYS = {
+  NOTIFICATIONS: 'app_notification_settings',
+} as const