Invite the people the message was already between

Follow-up to #167: an event made from a mail now opens with the sender
and everyone it was addressed to already in the guest list, so a thread
becomes a meeting without retyping the room.

Two things are deliberately left out. The reader's own addresses, since
they are the organiser and an organiser among their own guests is an
invitation to your own appointment. And a blind copy, on a message the
reader sent: a guest list is visible to every guest, so promoting a Bcc
to a guest would tell the room about a copy the sender chose to hide.
That is not something a menu item may do quietly.

*Send invitation emails to guests* now starts off when the guests were
inherited rather than typed, and on everywhere else -- which is every
event whose guests somebody chose one at a time. The reason is the case
the issue opened with: a reminder made out of a bill carries the biller
and everyone else on the mail. Left on, the primary button reads Send
invites and the first press mails all of them an invitation to what was
meant as a note to self. The switch sits right there under the list and
says what it does, so inviting them is one deliberate click. Un-sending
is not.

web/src/lib/appointment.ts

@@ -1,6 +1,7 @@
-import type { Email } from "@/jmap/types";
+import type { Email, EmailAddress } from "@/jmap/types";
 import { useCalendar, type EventDraft } from "@/store/calendar";
 import { useMail } from "@/store/mail";
+import { uniqueAddresses } from "./address";
 import { toLocalDateOnly } from "./dates";
 import { htmlToText } from "./text";
 
@@ -48,7 +49,23 @@ function bodyText(email: Email): string {
  * so the editor opens with the reader's cursor on a form they finish, rather
  * than a guess they have to check.
  */
-export function appointmentDraft(email: Email, now: Date = new Date()): EventDraft {
+/**
+ * Everyone the message was between, as guests: the sender and the people it
+ * was addressed to.
+ *
+ * The reader's own addresses come out -- they are the organiser, and an
+ * organiser listed among their own guests is an event that invites you to your
+ * own appointment. Bcc stays out too, on a message the reader sent themselves:
+ * a blind recipient added to a guest list is visible to every other guest, and
+ * turning a hidden copy into a public one is not something a menu item should
+ * do quietly.
+ */
+function guests(email: Email, ownEmails: string[]): EmailAddress[] {
+  const own = new Set(ownEmails.map((e) => e.toLowerCase()));
+  return uniqueAddresses([...(email.from ?? []), ...(email.to ?? []), ...(email.cc ?? [])]).filter((a) => !own.has(a.email.trim().toLowerCase()));
+}
+
+export function appointmentDraft(email: Email, now: Date = new Date(), ownEmails: string[] = []): EventDraft {
   const start = nextHalfHour(now);
   const body = bodyText(email).trim();
   return {
@@ -57,6 +74,7 @@ export function appointmentDraft(email: Email, now: Date = new Date()): EventDra
     start,
     end: new Date(start.getTime() + 3600_000),
     allDay: false,
+    attendees: guests(email, ownEmails),
   };
 }
 
@@ -68,8 +86,12 @@ export function appointmentDraft(email: Email, now: Date = new Date()): EventDra
  * has already been read.
  */
 export async function startAppointment(email: Email, navigate: (to: string) => void): Promise<void> {
-  const full = (await useMail.getState().getEmails([email.id], true))[0] ?? email;
-  const draft = appointmentDraft(full);
+  const mail = useMail.getState();
+  const full = (await mail.getEmails([email.id], true))[0] ?? email;
+  // Which addresses are the reader's own decides who is a guest, so they are
+  // worth a round trip when the session has not loaded them yet.
+  const identities = mail.identities.length ? mail.identities : await mail.loadIdentities();
+  const draft = appointmentDraft(full, new Date(), identities.map((i) => i.email));
   useCalendar.getState().setDraft(draft);
   navigate(`/calendar/day/${toLocalDateOnly(draft.start)}`);
 }