Calendar: edit and delete a single occurrence

Closes #132.

Stalwart 0.16.20 accepts a synthetic id on `CalendarEvent/set`, writing a
`recurrenceOverrides` entry rather than touching the series, so editing
one date of a recurring event is now something the server does and this
does too.

Editing asks the scope *before* the form opens, because it decides which
event the form is even about: a form populated from the master shows the
series' start date, so editing Wednesday's standup would have offered to
move Monday's. Deleting asks in place of the old confirm.

The patch is narrowed rather than posted hopefully. 0.16.20 sorts
per-occurrence properties into three groups and only one is honest: ten
are refused with `invalidProperties`, twelve more are dropped from the
patch while the response still reports success, and the rest are applied.
That silent middle group is how #26 reached a live server - a successful
response is not evidence anything was written - so `occurrencePatch`
throws on the first group, reports the second to the caller, and the
editor leaves out the five it always sends. A patch that would be
entirely dropped is not sent at all.

The refusal for an occurrence of a this-and-future change offers the
series instead of a bare error toast. Nothing here writes one of those,
but an event synced from another client can carry one.

Two things the scope prompt cost, both worth knowing. A dialog is queued
in a store the moment it is asked for, so it outlives the effect that
asked: without a ref guard a remount queues a second prompt the first
answer cannot retract. And gating the *answer* on the effect's cleanup
flag is worse - StrictMode runs mount, cleanup, mount, so the flag is
already set by the time anyone clicks and the editor never opens.

The mock expands recurrences for the first time, which is what makes any
of this developable. It hands out synthetic ids for everything including
one-offs, gives occurrences a `recurrenceId` and no rule, and reproduces
the refusals - including the silent drops, since a mock that applied them
would let a client that sends them look correct everywhere but a real
server.

server/src/mock/recurrence.test.ts

@@ -0,0 +1,133 @@
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+import { expandOccurrences, occurrenceAt, occurrenceView, parseSyntheticId, splitOccurrencePatch, syntheticId } from "./recurrence.js";
+
+/**
+ * The mock expands recurrences so that per-occurrence editing can be developed
+ * against something. What it has to get right is not the expansion — that is
+ * the easy half — but the three things a live server does that a client will
+ * otherwise be written against wrongly:
+ *
+ * - every expanded id is synthetic, one-offs included;
+ * - an occurrence carries a `recurrenceId` and no rule;
+ * - a per-occurrence patch loses some properties in silence.
+ */
+
+const WEEKDAYS = { "@type": "RecurrenceRule", frequency: "weekly", byDay: [{ day: "mo" }, { day: "tu" }, { day: "we" }, { day: "th" }, { day: "fr" }] };
+
+/** A standup at 09:00 every weekday, starting Monday 2026-09-07. */
+const series = () => ({ id: "ev1", "@type": "Event", uid: "u1", title: "Standup", start: "2026-09-07T09:00:00", duration: "PT30M", recurrenceRule: WEEKDAYS } as Record<string, unknown>);
+const oneOff = () => ({ id: "ev2", "@type": "Event", uid: "u2", title: "Lunch", start: "2026-09-08T12:00:00", duration: "PT1H" } as Record<string, unknown>);
+
+const week = (from: string, to: string) => [new Date(from), new Date(to)] as const;
+
+describe("expandOccurrences", () => {
+  it("gives a weekday rule five dates in a week and skips the weekend", () => {
+    const [a, b] = week("2026-09-07T00:00:00", "2026-09-14T00:00:00");
+    const out = expandOccurrences(series(), a, b);
+    assert.deepEqual(out.map((o) => o.start), [
+      "2026-09-07T09:00:00", "2026-09-08T09:00:00", "2026-09-09T09:00:00",
+      "2026-09-10T09:00:00", "2026-09-11T09:00:00",
+    ]);
+  });
+
+  it("gives a one-off exactly one occurrence, at index 0", () => {
+    const [a, b] = week("2026-09-01T00:00:00", "2026-10-01T00:00:00");
+    const out = expandOccurrences(oneOff(), a, b);
+    assert.equal(out.length, 1);
+    assert.equal(out[0]!.index, 0);
+  });
+
+  it("honours count", () => {
+    const ev = { ...series(), recurrenceRule: { ...WEEKDAYS, count: 3 } };
+    const [a, b] = week("2026-09-07T00:00:00", "2026-10-01T00:00:00");
+    assert.equal(expandOccurrences(ev, a, b).length, 3);
+  });
+
+  it("skips an excluded date but does not renumber the ones after it", () => {
+    // The whole reason an index rather than a position is the id: deleting
+    // Tuesday must not turn Wednesday's id into Tuesday's.
+    const ev = { ...series(), recurrenceOverrides: { "2026-09-08T09:00:00": { excluded: true } } };
+    const [a, b] = week("2026-09-07T00:00:00", "2026-09-14T00:00:00");
+    const out = expandOccurrences(ev, a, b);
+    assert.deepEqual(out.map((o) => o.start), [
+      "2026-09-07T09:00:00", "2026-09-09T09:00:00", "2026-09-10T09:00:00", "2026-09-11T09:00:00",
+    ]);
+    // Wednesday is still index 2, as it was before Tuesday went.
+    assert.equal(out[1]!.index, 2);
+    assert.equal(occurrenceAt(ev, 2)!.start, "2026-09-09T09:00:00");
+  });
+
+  it("carries an override onto the occurrence it keys", () => {
+    const ev = { ...series(), recurrenceOverrides: { "2026-09-09T09:00:00": { title: "Standup (long)" } } };
+    const [a, b] = week("2026-09-07T00:00:00", "2026-09-14T00:00:00");
+    const out = expandOccurrences(ev, a, b);
+    assert.deepEqual(out.find((o) => o.start === "2026-09-09T09:00:00")!.override, { title: "Standup (long)" });
+  });
+});
+
+describe("occurrenceView", () => {
+  it("strips the rule, sets recurrenceId, and points baseEventId at the master", () => {
+    const base = series();
+    const occ = occurrenceAt(base, 1)!;
+    const view = occurrenceView(base, occ);
+    assert.equal(view.id, syntheticId("ev1", 1));
+    assert.equal(view.baseEventId, "ev1");
+    assert.equal(view.recurrenceId, "2026-09-08T09:00:00");
+    assert.equal(view.recurrenceRule, undefined);
+    assert.equal(view.recurrenceOverrides, undefined);
+  });
+
+  it("gives a one-off a synthetic id over a different base, and no recurrenceId", () => {
+    // Both halves matter. The id is why `baseEventId` proves nothing about a
+    // series; the absent `recurrenceId` is why a one-off does not read as one.
+    const base = oneOff();
+    const view = occurrenceView(base, occurrenceAt(base, 0)!);
+    assert.equal(view.id, "ev2-o0");
+    assert.equal(view.baseEventId, "ev2");
+    assert.notEqual(view.id, view.baseEventId);
+    assert.equal(view.recurrenceId, undefined);
+  });
+
+  it("lets an override win over the series", () => {
+    const base = { ...series(), recurrenceOverrides: { "2026-09-08T09:00:00": { title: "Moved" } } };
+    const view = occurrenceView(base, occurrenceAt(base, 1)!);
+    assert.equal(view.title, "Moved");
+  });
+});
+
+describe("parseSyntheticId", () => {
+  it("round-trips", () => {
+    assert.deepEqual(parseSyntheticId(syntheticId("ev1", 12)), { baseId: "ev1", index: 12 });
+  });
+  it("does not claim a stored id", () => {
+    assert.equal(parseSyntheticId("ev1"), null);
+  });
+});
+
+describe("splitOccurrencePatch", () => {
+  it("applies what an occurrence takes", () => {
+    const { rejected, applied } = splitOccurrencePatch({ title: "Just today", color: "#f00" });
+    assert.equal(rejected, undefined);
+    assert.deepEqual(applied, { title: "Just today", color: "#f00" });
+  });
+
+  it("refuses an event-level property by name", () => {
+    assert.equal(splitOccurrencePatch({ calendarIds: { c2: true } }).rejected, "calendarIds");
+    assert.equal(splitOccurrencePatch({ hideAttendees: true }).rejected, "hideAttendees");
+  });
+
+  it("drops an inherited property in silence, which is the dangerous half", () => {
+    // No `rejected`, nothing applied, and a real server would still answer
+    // "updated". Anything that trusts the response believes this landed.
+    const { rejected, applied } = splitOccurrencePatch({ privacy: "private", recurrenceRule: null });
+    assert.equal(rejected, undefined);
+    assert.deepEqual(applied, {});
+  });
+
+  it("judges a pointer patch on its first token", () => {
+    assert.deepEqual(splitOccurrencePatch({ "participants/me/participationStatus": "accepted" }).applied,
+      { "participants/me/participationStatus": "accepted" });
+    assert.deepEqual(splitOccurrencePatch({ "participants/me/calendarAddress": "mailto:x@y" }).applied, {});
+  });
+});