Calendars page: refresh the kept list after Back, and batch the peek's event reads #968

These follow-ups come from the review of the Calendars page opening animation (#958).

1. The list is out of date after Back. When a calendar is opened from its card, the list is kept so Back can return to it instantly. If the owner changes the current date or adds events in the calendar and then presses Back, the card still shows the old date and the old peek marks until the page is reloaded. Fix: after Back, re-fetch the list, or at least that card's slot.
2. The list costs more queries per card. The peek reads each card's current month of events (ListEventsForMonth in list_handler.go), which adds several queries per calendar. That is fine for a few calendars but slow for dozens. Fix: a batched month-events read, or skip the peek beyond a set number of cards.

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