Stop calling every event a series

A one-time event opened for editing said "this is a recurring event —
changes apply to the whole series", and deleting one offered to delete
all occurrences of an event that has exactly one.

Three places asked whether an event had a baseEventId and took that for
recurrence. It isn't: the calendar loads its range with expandRecurrences,
and Stalwart puts a baseEventId on everything it returns that way, a
one-off pointing at itself included. The mock never sets the field at
all, which is why this only showed up against a real server.

They now share isRecurring(), which asks about recurrence rules, and
treats a base that is some other event as an occurrence of a series too
— so an expanded instance that travels without its rules is still
described honestly on the way to being deleted.

Fixes #25

web/src/store/calendar.ts

@@ -297,6 +297,20 @@ export const useCalendar = create<CalendarState>((set, get) => ({
   },
 }));
 
+/**
+ * Whether an event is part of a series.
+ *
+ * Not the same question as "does it have a baseEventId": `CalendarEvent/query`
+ * runs with `expandRecurrences`, and Stalwart sets `baseEventId` on every event
+ * it returns that way — a one-off event included, pointing at itself. Recurrence
+ * rules are what make a series, so those are the question; a base that is some
+ * *other* event means this is one occurrence of one, whether or not the expanded
+ * instance carried the rules along with it.
+ */
+export function isRecurring(ev: CalendarEvent): boolean {
+  return Boolean(ev.recurrenceRules?.length || ev.excludedRecurrenceRules?.length || (ev.baseEventId && ev.baseEventId !== ev.id));
+}
+
 export function toInstance(e: CalendarEvent, calendars: Record<Id, Calendar>): EventInstance | null {
   const allDay = Boolean(e.showWithoutTime);
   let start: Date;