feat: per-admin localStorage namespacing with migration

Made-with: Cursor

frontend/src/utils/adminPrefs.js

@@ -0,0 +1,19 @@
+const PREF_KEYS = ['history-filter', 'history-show-limit'];
+
+export function prefixKey(adminName, key) {
+  if (!adminName) return key;
+  return `${adminName}:${key}`;
+}
+
+export function migratePreferences(adminName) {
+  if (!adminName) return;
+
+  for (const key of PREF_KEYS) {
+    const oldValue = localStorage.getItem(key);
+    const newKey = prefixKey(adminName, key);
+    if (oldValue !== null && localStorage.getItem(newKey) === null) {
+      localStorage.setItem(newKey, oldValue);
+      localStorage.removeItem(key);
+    }
+  }
+}