fix(vacay): fix entitlement counter, year deletion, and year creation bugs

- toggleCompanyHoliday now calls loadStats() so the entitlement sidebar
  updates immediately when a vacation day is converted to a company holiday
- deleteYear now deletes vacay_user_years rows for the removed year,
  preventing stale entitlement data from persisting and re-appearing
  when the year is re-created
- deleteYear recalculates carry-over for year+1 when year N is deleted,
  using the new actual previous year as the source
- removeYear store action now calls loadStats() so the sidebar reflects
  the recalculated carry-over without requiring a page refresh
- Add prev-year button (+[<] 2026 [>]+) so users can add years going
  backwards after deleting a past year; add vacay.addPrevYear i18n key
  to all 13 supported languages

Closes #371

client/src/store/vacayStore.ts

@@ -229,6 +229,7 @@ export const useVacayStore = create<VacayState>((set, get) => ({
         : new Date().getFullYear()
     }
     set(updates)
+    await get().loadStats()
   },
 
   loadEntries: async (year?: number) => {
@@ -246,6 +247,7 @@ export const useVacayStore = create<VacayState>((set, get) => ({
   toggleCompanyHoliday: async (date: string) => {
     await api.toggleCompanyHoliday(date)
     await get().loadEntries()
+    await get().loadStats()
   },
 
   loadStats: async (year?: number) => {