feat: Sondervertretungen als eigene HR-Seite (/hr/special-assignments)

- Neue Seite SpecialAssignmentsPage mit Filter, Tabelle, Add/Edit-Modal
- Farbcodierung: Faktor >1.0 amber, <1.0 blau, =1.0 grau
- Monat-Filterung client-seitig, paralleles Laden in Batches
- Layout.tsx: Nav-Eintrag in Hauptnavigation
- App.tsx: Route /hr/special-assignments
- UsersPage: Sondervertretungs-Block aus Edit-Modal entfernt

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

frontend/src/App.tsx

@@ -26,6 +26,7 @@ import { KioskSetupPage } from './pages/KioskSetupPage'
 import { KioskStampPage } from './pages/KioskStampPage'
 import { MobilePage } from './pages/mobile/MobilePage'
 import { MobileLoginPage } from './pages/mobile/MobileLoginPage'
+import { SpecialAssignmentsPage } from './pages/SpecialAssignmentsPage'
 
 export default function App() {
   return (
@@ -57,6 +58,7 @@ export default function App() {
             <Route path='/settings/company'        element={<CompanySettingsPage />} />
             <Route path='/settings/kiosk'          element={<KioskDevicesPage />} />
             <Route path='/settings/audit-log'      element={<AuditLogPage />} />
+            <Route path='/hr/special-assignments'  element={<SpecialAssignmentsPage />} />
             <Route path='/profile'                 element={<ProfilePage />} />
           </Route>
           <Route path='*' element={<Navigate to='/login' replace />} />