feat: Abwesenheiten-Screen in Mobile-App

- MobileAbsencesScreen.tsx:
  - Urlaubskonto-Karte (verbleibende Tage + Fortschrittsbalken)
  - Liste eigener Abwesenheiten (aktuell/geplant + vergangen)
  - Farbpunkt pro Abwesenheitstyp, Status-Badge
  - Bottom-Sheet Modal: Antrag stellen oder Krank melden
  - Start-/Enddatum-Picker, Typ-Auswahl, optionale Notiz
  - SICK-Typ → quick-sick Endpoint, sonst POST /absences/
- MobileBottomNav: 4. Tab 'Urlaub' (war 3 Tabs)
- MobilePage: Screen 'absences' eingebunden

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

frontend/src/pages/mobile/MobileBottomNav.tsx

@@ -1,4 +1,4 @@
-type Screen = 'stamp' | 'today' | 'profile'
+type Screen = 'stamp' | 'today' | 'absences' | 'profile'
 
 interface Props {
   active: Screen
@@ -29,6 +29,16 @@ export function MobileBottomNav({ active, onChange }: Props) {
         </svg>
       ),
     },
+    {
+      id: 'absences',
+      label: 'Urlaub',
+      icon: (
+        <svg className='w-6 h-6' fill='none' viewBox='0 0 24 24' stroke='currentColor' strokeWidth={1.75}>
+          <path strokeLinecap='round' strokeLinejoin='round' d='M3 17l4-8 4 4 4-6 4 10' />
+          <path strokeLinecap='round' strokeLinejoin='round' d='M3 20h18' />
+        </svg>
+      ),
+    },
     {
       id: 'profile',
       label: 'Profil',
@@ -52,9 +62,7 @@ export function MobileBottomNav({ active, onChange }: Props) {
             key={item.id}
             onClick={() => onChange(item.id)}
             className={`flex-1 flex flex-col items-center justify-center gap-1 min-h-[56px] py-2 transition-colors ${
-              active === item.id
-                ? 'text-blue-600'
-                : 'text-gray-400'
+              active === item.id ? 'text-blue-600' : 'text-gray-400'
             }`}
           >
             {item.icon}