Add admin user creation & must-change flag

Add server and UI support for creating admin users and forcing password change. API: introduce CreateUserRequest contract and add CreateNewAppUser endpoint in AppUserController; extend ChangeUserRequest with MustChangePassword and handle role assignment and detailed error responses (409/422/400). Frontend: new CreateUserDialog component, integrate it into AdminUsers list, and add createAdminUser service with CreateAdminUserError and payload handling; include mustChangePassword in update payloads and EditUserDialog. UI polish: enhanced app banner enter/leave animations in Layout.vue and add auto-dismiss timers/cleanup to appBanners store to limit and auto-remove banners.

GUI/src/routes/admin/AdminUsers.vue

@@ -7,6 +7,7 @@ import { useAppBannersStore } from '@/stores/appBanners'
 import StatusPill from '@/components/ui/StatusPill.vue'
 import UserAvatar from '@/components/ui/UserAvatar.vue'
 import EmptyState from '@/components/ui/EmptyState.vue'
+import CreateUserDialog from '@/components/admin/CreateUserDialog.vue'
 
 const router = useRouter()
 const appBannersStore = useAppBannersStore()
@@ -15,6 +16,7 @@ const isLoading = ref(true)
 const errorMessage = ref('')
 const users = ref<AdminUser[]>([])
 const searchQuery = ref('')
+const isCreateDialogOpen = ref(false)
 
 const hasUsers = computed(() => users.value.length > 0)
 const activeUserCount = computed(() => users.value.filter((user) => user.isActive).length)
@@ -108,6 +110,14 @@ async function openUserDetail(userId: string) {
   await router.push({ name: 'AdminUserDetail', params: { userId } })
 }
 
+function openCreateDialog() {
+  isCreateDialogOpen.value = true
+}
+
+function handleUserCreated(user: AdminUser) {
+  users.value = [...users.value, user]
+}
+
 onMounted(() => {
   void loadUsers()
 })
@@ -135,6 +145,15 @@ onMounted(() => {
         />
         <v-btn
           variant="elevated"
+          color="primary"
+          prepend-icon="mdi-account-plus-outline"
+          :disabled="isLoading"
+          @click="openCreateDialog"
+        >
+          Benutzer anlegen
+        </v-btn>
+        <v-btn
+          variant="outlined"
           prepend-icon="mdi-refresh"
           :loading="isLoading"
           :disabled="isLoading"
@@ -303,6 +322,8 @@ onMounted(() => {
         </article>
       </div>
     </section>
+
+    <CreateUserDialog v-model="isCreateDialogOpen" @created="handleUserCreated" />
   </v-container>
 </template>