feat(auth): configure form login to use 'email' as username parameter

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/config/SecurityConfig.java

@@ -67,7 +67,7 @@ public class SecurityConfig {
                         .frameOptions(frameOptions -> frameOptions.sameOrigin()))
                 // Erlaubt Login via Browser-Popup oder REST-Header (Authorization: Basic ...)
                 .httpBasic(Customizer.withDefaults())
-                .formLogin(Customizer.withDefaults());
+                .formLogin(form -> form.usernameParameter("email"));
 
         return http.build();
     }