feat(leave) : prorate forfait work-target days in employee header

The header hardcoded '218 jours' and '218 - presence restants', wrong for a
forfait entered mid-year. Expose forfaitWorkTargetDays = businessDays(period) -
acquiredDays (218 full year, prorated otherwise) and show
'Forfait - {target} jours ({presence} présence · {target-presence} restants)'.
Grégory: 155 jours (11 présence · 144 restants).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/services/dto/employee-leave-summary.ts

@@ -16,6 +16,7 @@ export type EmployeeLeaveSummary = {
   previousYearPaidDays: number
   presenceDaysByMonth: Record<string, number>
   presenceDaysToToday: number
+  forfaitWorkTargetDays: number | null
   dataStartDate: string | null
 }