fix(17): IN-07 memoize Intl.DateTimeFormat timezone resolution

apps/pwa/src/routes/AdminPage.tsx

@@ -23,7 +23,7 @@
  * Security: client isAdmin gate is UX only. Server 403 is the real boundary (D-03).
  */
 
-import { useState, useRef, useEffect } from 'react';
+import { useState, useRef, useEffect, useMemo } from 'react';
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import { CheckCircle, AlertCircle, Loader2 } from 'lucide-react';
 import {
@@ -157,8 +157,9 @@ export function AdminPage() {
     },
   });
 
-  // Detected browser timezone (D-02)
-  const detectedTz = Intl.DateTimeFormat().resolvedOptions().timeZone;
+  // Detected browser timezone (D-02). IN-07: memoize — the resolved zone is
+  // stable for the session, no need to recompute every render.
+  const detectedTz = useMemo(() => Intl.DateTimeFormat().resolvedOptions().timeZone, []);
 
   // Effective timezone input value: local override → stored value → ''
   const storedTimezone = timezoneQuery.data?.timezone ?? '';