feat(absence) : extract Absence front into Nuxt module layer

LST-66 (2.3) front. Companion to the backend module migration.

- Move pages (absences, team-absences), 8 components, the absences service +
  DTO and the useAbsenceHelpers composable into frontend/modules/absence/
  (auto-detected layer; composable now auto-imported).
- Rewrite consumers: AdminAbsencePolicyTab and the time-tracking calendar
  (getPublicHolidays) point to ~/modules/absence/...
- Middlewares (employee/admin) and shared services (clients, users,
  user-data DTO) stay at the root. i18n stays global.
- Routes /absences and /team-absences preserved.

nuxt build passes; routes confirmed.

frontend/modules/time-tracking/components/TimeTrackingCalendar.vue

@@ -151,7 +151,7 @@
 
 <script setup lang="ts">
 import type { TimeEntry } from '~/modules/time-tracking/services/dto/time-entry'
-import { useAbsenceService } from '~/services/absences'
+import { useAbsenceService } from '~/modules/absence/services/absences'
 
 const { t } = useI18n()
 const absenceService = useAbsenceService()