feat(tenants): UI fuer Mandanten-Verwaltung — Modul 4/16 des UI-Backlogs

Backend existierte vollstaendig (list, create, list users, assign user;
tenants:read/write), Frontend hatte 0% Abdeckung.

- api/tenants.ts: TanStack-Hooks (useTenants, useTenantUsers mit
  enabled-Gating, create, assignUser mit Cache-Invalidierung)
- pages/Tenants.tsx: Tenant-Karten (Name, Slug, Standard-Badge),
  expandierbare User-Liste pro Tenant (Rolle, E-Mail), Create-Dialog
  (Name + Slug mit Auto-Normalisierung), Assign-User-Dialog mit
  User-Picker (bereits zugewiesene gefiltert) — Create/Users/Assign
  hinter tenants:write gegated
- Platzierung: Settings-Subpage /settings/tenants (statisch, Core-Route)
  + Settings-Nav-Item
- i18n tenants.* de/en

Verifikation: Vitest 8/8 (Rendering, Standard-Badge, Expand-User-Liste,
Create-Flow, Assign-Flow, Permission-Gating, Empty/Error) · tsc exit 0 ·
production build exit 0.

frontend/src/pages/Settings.tsx

@@ -49,6 +49,7 @@ export function SettingsPage() {
     { to: '/settings/workspaces', label: 'Workspaces', icon: '\ud83d\udd58\ufe0f' },
     { to: '/settings/backup', label: 'Backup & Restore', icon: '\ud83d\udcbe' },
     { to: '/settings/api-tokens', label: 'API-Tokens', icon: '\ud83d\udd11' },
+    { to: '/settings/tenants', label: 'Mandanten', icon: '\ud83c\udfe2' },
   ];
 
   const existingPaths = new Set(hardcodedNavItems.map(item => item.to));