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.test.tsx

@@ -839,5 +839,38 @@ describe('Settings', () => {
 
       expect(hooks.useCalendarDays).toHaveBeenLastCalledWith('ws-1', CURRENT_YEAR - 4, 4)
     })
+
+    it('wires the toggle button through to the shared day-overrides hook', async () => {
+      vi.mocked(hooks.useCalendarDays).mockReturnValue(
+        makeCalendarDaysResult({
+          data: {
+            ...mockCalendarWindow,
+            days: [
+              {
+                date: `${CURRENT_YEAR}-01-01`,
+                name: 'Nyårsdagen',
+                dayType: 'red_day',
+                isOverride: false,
+                overrideIsWorkingDay: null,
+                isFree: true,
+              },
+            ],
+          },
+        })
+      )
+
+      renderSettings()
+
+      fireEvent.click(screen.getByTestId(`red-days-toggle-${CURRENT_YEAR}-01-01`))
+
+      await waitFor(() => {
+        expect(mockUpsertDayOverride).toHaveBeenCalledWith({
+          workspaceId: 'ws-1',
+          date: `${CURRENT_YEAR}-01-01`,
+          isWorkingDay: true,
+          note: null,
+        })
+      })
+    })
   })
 })