Ask before deleting without leaving the app

Deleting a period or a day went through the browser's own confirmation box.
It halts the page while it is open, which meant the delete path could not be
driven by anything automated, and it looks nothing like the rest of the app.
Both now ask inline: the first click offers the choice, and only the second
one deletes. That matters more than it did, since deleting a period now takes
its days with it.

frontend/src/i18n/en/calendar.ts

@@ -40,6 +40,7 @@ export const calendar = {
   'plannedDayModal.closeAria': 'Close',
   'plannedDayModal.confirmDelete': 'Are you sure you want to delete this planned day?',
   'plannedDayModal.deleteButton': 'Delete',
+  'plannedDayModal.deleteConfirmButton': 'Yes, delete',
   'plannedDayModal.deleteError': 'Failed to delete planned day',
   'plannedDayModal.deletingButton': 'Deleting…',
   'plannedDayModal.doubleDayHint': 'Double days do not apply to vacation',
@@ -108,6 +109,7 @@ export const calendar = {
   'calendar.periodDeleteButton': 'Delete period',
   'calendar.periodDeletingButton': 'Deleting…',
   'calendar.periodDeleteConfirm': 'Are you sure you want to delete this period?',
+  'calendar.periodDeleteConfirmButton': 'Yes, delete',
   'calendar.periodDeleteAdHocButton': 'Delete these days',
   'calendar.periodCancelButton': 'Cancel',
   'calendar.periodCloseAria': 'Close',