feat(ui): Gäste-Verwaltung UI — SettingsGuests (UI-Backlog Modul 14/16)

- api/guests.ts: useGuests, useInviteGuest, useRevokeGuest (/api/v1/guests)
- SettingsGuests.tsx: Admin-Gate (Outbox-Muster), Gästeliste mit Status-Badges
  (invited/active/disabled), Invite-Modal (RHF+zod), Revoke-ConfirmDialog
- Route /settings/guests + Nav-Eintrag in Settings.tsx
- i18n de/en: 14 Keys
- Vitest 8/8, tsc clean, Build OK

frontend/src/api/guests.ts

@@ -0,0 +1,65 @@
+/**
+ * Guests API client — admin guest management (UI-Backlog module 14/16).
+ *
+ * Backend: /api/v1/guests (app/routes/guests.py, require_admin).
+ * - GET    /           → list guests (role=guest memberships for tenant)
+ * - POST   /invite     → invite guest (name + email; 409 if already active)
+ * - DELETE /{id}       → revoke guest (status=disabled + session invalidation)
+ *
+ * Guests are regular users with role='guest' in user_tenants; they log in
+ * via the normal login flow (password via reset flow).
+ */
+
+import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
+import { apiGet, apiPost, apiDelete } from '@/api/client';
+
+export interface Guest {
+  id: string;
+  email: string;
+  name: string;
+  status: 'invited' | 'active' | 'disabled' | string;
+  role: string;
+  created_at: string | null;
+}
+
+export interface GuestInviteResponse {
+  email: string;
+  name: string;
+  status: string;
+  role: string;
+  message: string;
+}
+
+export interface GuestInvitePayload {
+  email: string;
+  name: string;
+}
+
+export function useGuests(enabled = true) {
+  return useQuery({
+    queryKey: ['guests'],
+    queryFn: () => apiGet<Guest[]>('/guests'),
+    enabled,
+  });
+}
+
+export function useInviteGuest() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: (data: GuestInvitePayload) =>
+      apiPost<GuestInviteResponse>('/guests/invite', data),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['guests'] });
+    },
+  });
+}
+
+export function useRevokeGuest() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: (guestId: string) => apiDelete<{ message: string }>(`/guests/${guestId}`),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['guests'] });
+    },
+  });
+}