chore(types): replace deprecated FormEvent with SyntheticEvent

React 19's @types/react flags FormEvent as deprecated — quote: 'FormEvent
doesn't actually exist. You probably meant to use ChangeEvent, InputEvent,
SubmitEvent, or just SyntheticEvent'. All 17 call sites in this repo
typed form onSubmit handlers and only called e.preventDefault(), so
SyntheticEvent is the correct general replacement.

No behavior change — pure type-level cleanup that clears the chronic
deprecation hint that has been showing up after every edit to any
form-bearing page.

src/pages/AdminBulkActions.tsx

@@ -1704,7 +1704,7 @@ export default function AdminBulkActions() {
     return () => clearTimeout(searchTimerRef.current);
   }, []);
 
-  const handleSearchSubmit = (e: React.FormEvent) => {
+  const handleSearchSubmit = (e: React.SyntheticEvent) => {
     e.preventDefault();
     clearTimeout(searchTimerRef.current);
     setOffset(0);