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/__tests__/recurrence.test.ts

@@ -0,0 +1,27 @@
+import { describe, expect, it } from "vitest";
+import { isRecurring } from "@/store/calendar";
+import type { CalendarEvent } from "@/jmap/types";
+
+/**
+ * `CalendarEvent/query` runs with `expandRecurrences`, and Stalwart puts a
+ * `baseEventId` on everything it returns that way — one-off events included.
+ * Treating that as proof of a series told people editing a plain event that
+ * their changes applied to the whole series, and offered to delete "all
+ * occurrences" of an event that has exactly one.
+ */
+const ev = (p: Partial<CalendarEvent>): CalendarEvent => ({ id: "ev1", "@type": "Event", uid: "u1", calendarIds: { c1: true }, start: "2026-08-25T09:00:00", duration: "PT30M", ...p } as CalendarEvent);
+
+describe("isRecurring", () => {
+  it("does not call a one-off event a series just because it has a baseEventId", () => {
+    expect(isRecurring(ev({ baseEventId: "ev1" }))).toBe(false);
+    expect(isRecurring(ev({}))).toBe(false);
+  });
+  it("still recognises an occurrence whose base is another event", () => {
+    expect(isRecurring(ev({ id: "ev1_2", baseEventId: "ev1" }))).toBe(true);
+  });
+  it("recognises a series by its recurrence rules", () => {
+    expect(isRecurring(ev({ recurrenceRules: [{ "@type": "RecurrenceRule", frequency: "weekly" }] }))).toBe(true);
+    expect(isRecurring(ev({ baseEventId: "ev1", recurrenceRules: [{ "@type": "RecurrenceRule", frequency: "daily" }] }))).toBe(true);
+    expect(isRecurring(ev({ excludedRecurrenceRules: [{ "@type": "RecurrenceRule", frequency: "monthly" }] }))).toBe(true);
+  });
+});