fix(gallery): clear the guest identity on gallery logout

The gallery password is one shared secret per event and does not
distinguish people. With the guest identity outliving the tab, logging out
and letting the next person enter that password greeted them by the
previous guest's name, with "forget me" - which erases that guest's
selections server-side - one click away. Logout is the leaving-this-device
signal, so it now drops the local identity too. Server row untouched.

frontend/src/contexts/GalleryAuthContext.tsx

@@ -11,6 +11,7 @@ import {
   setActiveGallerySlug,
   storeGalleryToken,
 } from '../utils/galleryAuthStorage';
+import { clearGuestIdentity } from '../utils/guestIdentityStorage';
 import type { GalleryAccessLevel } from '../types';
 
 interface GalleryEvent {
@@ -350,6 +351,12 @@ export const GalleryAuthProvider: React.FC<GalleryAuthProviderProps> = ({ childr
       sessionStorage.removeItem(`gallery_event_${currentSlug}`);
       sessionStorage.removeItem(`gallery_access_level_${currentSlug}`);
       clearGalleryToken(currentSlug);
+      // Logout is the "I am leaving this device" signal. Now that the guest
+      // identity outlives the tab (#1265), leaving it behind would greet the
+      // next person who enters the shared gallery password by this guest's
+      // name — with "forget me", which erases THIS guest's selections
+      // server-side, one click away. Local only; the server row is untouched.
+      clearGuestIdentity(currentSlug);
     }
     authService.galleryLogout(currentSlug || undefined);
     setIsAuthenticated(false);