feat: New users require admin approval

- Set active=false for newly registered users
- Return RegistrationResponse instead of SessionResponse after registration
- Show success message informing user that admin approval is pending
- Login check already filters for active users only

backend/src/main/java/de/svencarstensen/muh/service/CatalogService.java

@@ -581,7 +581,7 @@ public class CatalogService {
                 normalizedEmail,
                 phoneNumber,
                 passwordEncoder.encode(mutation.password()),
-                true,
+                false,
                 UserRole.CUSTOMER,
                 100000L,
                 now,
@@ -601,13 +601,16 @@ public class CatalogService {
                 created.email(),
                 created.phoneNumber(),
                 created.passwordHash(),
-                created.active(),
+                false,
                 created.role(),
                 created.nextSampleNumber(),
                 created.createdAt(),
                 created.updatedAt()
         ));
-        return toSessionResponse(accountBound);
+        return new RegistrationResponse(accountBound.id(), accountBound.email());
+    }
+
+    public record RegistrationResponse(String userId, String email) {
     }
 
     public UserOption currentUser(String actorId) {

backend/src/main/java/de/svencarstensen/muh/service/SampleService.java

@@ -28,7 +28,6 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
-import java.util.Optional;
 import java.util.stream.Collectors;
 
 @Service

backend/src/main/java/de/svencarstensen/muh/web/SessionController.java

@@ -56,7 +56,7 @@ public class SessionController {
     }
 
     @PostMapping("/register")
-    public CatalogService.SessionResponse register(@RequestBody RegistrationRequest request) {
+    public CatalogService.RegistrationResponse register(@RequestBody RegistrationRequest request) {
         return catalogService.registerCustomer(new CatalogService.RegistrationMutation(
                 request.companyName(),
                 request.street(),