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/pages/Settings.tsx

@@ -692,6 +692,10 @@ export function Settings(): ReactNode {
             error={calendarError}
             onPrevWindow={handleRedDaysPrevWindow}
             onNextWindow={handleRedDaysNextWindow}
+            overrides={dayOverrides}
+            workspaceId={workspace?.id ?? ''}
+            onUpsertOverride={upsertDayOverride}
+            onRemoveOverride={removeDayOverride}
           />
         </section>