Say it in the words Stalwart 0.16 answers to

Guests added to an event vanished on save and no invitation was ever
sent. Not a guard in the editor, and nothing the server complained
about: ihasmail addresses a participant the way RFC 8984 does, with
sendTo and email, and Stalwart 0.16 keeps that address under
calendarAddress. Handed the RFC's spelling it stores the event, drops
the entire participant map, and reports success. Six shapes were tried
against a live 0.16.19, down to sendTo and roles alone; all six were
dropped, and patching a participant onto an existing event fails
outright with "Patch operation failed".

The same disagreement runs through two more properties. The organizer is
organizerCalendarAddress, not replyTo. A recurrence is a single
recurrenceRule, not a recurrenceRules array — and that one Stalwart
refuses honestly, with invalidProperties, so no recurring event could be
created at all and existing ones showed no repeat.

So writes now use Stalwart's names and reads accept either, since a
mailbox may hold events written by other clients. The mock now refuses
what the real server refuses and drops what it drops: advertising the
RFC spelling is exactly how this reached a live server unnoticed, the
same way the capability-placement bug did.

Verified against 0.16.19: participants, organizer and rule all survive a
create, an update and a re-read, with the roles kept as sent.

Fixes #26
Fixes #30

web/src/store/calendar.ts

@@ -1,6 +1,6 @@
 import { create } from "zustand";
 import { CAP, client, setErrorMessage } from "@/jmap/client";
-import type { BusyPeriod, Calendar, CalendarEvent, GetResponse, Id, ParticipantIdentity, QueryResponse, SetResponse } from "@/jmap/types";
+import type { BusyPeriod, Calendar, CalendarEvent, GetResponse, Id, JSCalendarParticipant, JSCalendarRecurrenceRule, ParticipantIdentity, QueryResponse, SetResponse } from "@/jmap/types";
 import { toUTCDate, toLocalDateTime, zonedToDate, parseDuration, DAY_MS, browserTimeZone } from "@/lib/dates";
 import { settings } from "./settings";
 import { useSession } from "./session";
@@ -57,7 +57,7 @@ const EVENT_PROPS = [
   "id", "baseEventId", "calendarIds", "isDraft", "isOrigin", "utcStart", "utcEnd", "useDefaultAlerts", "mayInviteSelf", "mayInviteOthers", "hideAttendees",
   "uid", "relatedTo", "prodId", "created", "updated", "sequence", "title", "description", "descriptionContentType", "showWithoutTime",
   "locations", "virtualLocations", "links", "locale", "keywords", "categories", "color", "recurrenceId", "recurrenceIdTimeZone",
-  "recurrenceRules", "excludedRecurrenceRules", "recurrenceOverrides", "excluded", "priority", "freeBusyStatus", "privacy", "replyTo",
+  "recurrenceRules", "recurrenceRule", "excludedRecurrenceRules", "recurrenceOverrides", "excluded", "priority", "freeBusyStatus", "privacy", "replyTo", "organizerCalendarAddress",
   "sentBy", "participants", "requestStatus", "alerts", "timeZone", "start", "duration", "status",
 ];
 
@@ -338,6 +338,52 @@ export function toInstance(e: CalendarEvent, calendars: Record<Id, Calendar>): E
   return { key: e.id, event: e, start, end, allDay, calendar: calId ? calendars[calId] : undefined };
 }
 
+/**
+ * Every address a participant answers to, as lowercase `mailto:` URIs.
+ *
+ * Stalwart 0.16 keeps one address under `calendarAddress`; RFC 8984 spreads it
+ * over `sendTo` and `email`. Reading has to accept all three — a mailbox may
+ * hold events written by either, and by other clients besides.
+ */
+export function participantAddresses(p: JSCalendarParticipant): string[] {
+  return [p.calendarAddress ?? "", ...Object.values(p.sendTo ?? {}), p.email ? `mailto:${p.email}` : ""]
+    .filter(Boolean)
+    .map((a) => a.toLowerCase());
+}
+
+/** The address to show or write to, without the `mailto:`. */
+export function participantEmail(p: JSCalendarParticipant): string {
+  return (participantAddresses(p)[0] ?? "").replace(/^mailto:/i, "");
+}
+
+/** Whether this participant is attending, under any of the role names in use. */
+export function isAttendee(p: JSCalendarParticipant): boolean {
+  return Boolean(p.roles?.attendee || p.roles?.required || p.roles?.optional || p.roles?.chair);
+}
+
+/** The event's recurrence rule, under either spelling. */
+export function eventRule(ev: CalendarEvent): JSCalendarRecurrenceRule | undefined {
+  return ev.recurrenceRule ?? ev.recurrenceRules?.[0];
+}
+
+/**
+ * Builds a participant the way Stalwart 0.16 stores them: the address under
+ * `calendarAddress`. Sent under RFC 8984's `sendTo`/`email` instead, the server
+ * keeps the event and drops the whole participant map without saying so — which
+ * is how invitations came to vanish (#26).
+ */
+export function makeParticipant(email: string, name: string | null | undefined, role: "owner" | "attendee", status?: string): JSCalendarParticipant {
+  return {
+    "@type": "Participant",
+    name: name || undefined,
+    calendarAddress: `mailto:${email}`,
+    kind: "individual",
+    roles: role === "owner" ? { owner: true, attendee: true } : { attendee: true, required: true },
+    participationStatus: (status as JSCalendarParticipant["participationStatus"]) ?? (role === "owner" ? "accepted" : "needs-action"),
+    expectReply: role !== "owner",
+  };
+}
+
 export function myParticipantKeys(ev: CalendarEvent, identities: ParticipantIdentity[]): string[] {
   const mine = new Set<string>();
   for (const i of identities) {
@@ -348,8 +394,7 @@ export function myParticipantKeys(ev: CalendarEvent, identities: ParticipantIden
   if (session?.username?.includes("@")) mine.add(`mailto:${session.username.toLowerCase()}`);
   const keys: string[] = [];
   for (const [k, p] of Object.entries(ev.participants ?? {})) {
-    const addrs = [...Object.values(p.sendTo ?? {}), p.email ? `mailto:${p.email}` : ""].map((a) => a.toLowerCase());
-    if (addrs.some((a) => mine.has(a))) keys.push(k);
+    if (participantAddresses(p).some((a) => mine.has(a))) keys.push(k);
   }
   return keys;
 }