Take the time off together, in long stretches

A season preset in a two-parent household now means both parents, not
either one — being off together is the point of the period.

Within such a period the planner used to pick whoever had the most
parental-benefit days left, date by date, so spending kept moving the
lead and the funding flipped almost daily. Six weeks came out as a
patchwork that costs nothing to hold but a great deal to request. The
payer is now chosen once per half of the window, reusing the same split
the swap strategy already uses: one parent on parental benefit while the
other takes vacation, then they change over. Both are still off every
day, and if one runs out mid-stretch the roles swap for the rest rather
than day by day.

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

frontend/src/pages/Calendar.tsx

@@ -719,7 +719,7 @@ export function Calendar(): ReactNode {
         recurrence: 'yearly',
         firstYear: new Date().getFullYear(),
         priority: wishesHook.data.length + 1,
-        parentMode: 'either',
+        parentMode: membersHook.data.length >= 2 ? 'both' : 'either',
         windowMode: 'flexible',
       })