Flip a single holiday to a working day by clicking it

The status on the right of each public holiday is now a button. Clicking
it writes an override for that one date, and clicking it back removes the
override rather than storing a row that agrees with the almanac. Because
an override is keyed on an absolute date, the same holiday in other years
is left alone — which is what was asked for and needed no schema change.

The separate badge saying an override exists stays, so the fact that a
day has been changed is not carried by the button alone.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ei7gCZzdd751uZtPzb1Pep

frontend/src/i18n/en/calendar.ts

@@ -67,6 +67,9 @@ export const calendar = {
   'redDaysList.overrideFree': 'Exception: day off',
   'redDaysList.overrideWorking': 'Exception: working day',
   'redDaysList.prevWindow': 'Previous 4 years',
+  'redDaysList.toggleError': 'Failed to update this day. Try again.',
+  'redDaysList.toggleToFreeAria': 'Mark {date} as a day off',
+  'redDaysList.toggleToWorkingAria': 'Mark {date} as a working day',
 
   // §Task7 — the calendar becomes the plan: drag-select, period popover,
   // preset rail, and the sync toast/undo.