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/views/calendar/CalendarView.tsx

@@ -77,8 +77,8 @@ export function CalendarView({ view: viewParam, date }: { view?: string; date?:
    */
   useEffect(() => {
     if (!cal.draft) return;
-    const { title, description, ...when } = cal.draft;
-    setEditor({ ...when, seed: { title, description } });
+    const { title, description, attendees, ...when } = cal.draft;
+    setEditor({ ...when, seed: { title, description, attendees } });
     cal.setDraft(null);
   }, [cal.draft]);
 

web/src/views/calendar/EventEditor.tsx

@@ -27,7 +27,7 @@ export interface EditorInit {
    * so far. Not an event: this is still a form the reader has to finish, so
    * `editing` stays false and the dialog says New event / Create.
    */
-  seed?: { title?: string; description?: string };
+  seed?: { title?: string; description?: string; attendees?: EmailAddress[] };
 }
 
 const ALERT_OPTIONS = [0, 5, 10, 15, 30, 60, 120, 1440, 2880, 10080];
@@ -127,12 +127,25 @@ function EventForm({ init, base, scope, editing, onClose, settingsTz, defaultAle
   });
   const myKeys = ev ? myParticipantKeys(ev, cal.identities) : [];
   const [attendees, setAttendees] = useState<EmailAddress[]>(() =>
-    Object.entries(ev?.participants ?? {})
-      .filter(([k, p]) => !myKeys.includes(k) && !(p.roles?.owner && !p.roles?.attendee))
-      .map(([, p]) => ({ name: p.name ?? null, email: participantEmail(p) }))
-      .filter((a) => a.email),
+    ev
+      ? Object.entries(ev.participants ?? {})
+          .filter(([k, p]) => !myKeys.includes(k) && !(p.roles?.owner && !p.roles?.attendee))
+          .map(([, p]) => ({ name: p.name ?? null, email: participantEmail(p) }))
+          .filter((a) => a.email)
+      : (init.seed?.attendees ?? []),
   );
-  const [sendInvites, setSendInvites] = useState(true);
+  /*
+   * Off when the guest list was not typed but inherited -- from a message, so
+   * far -- and on everywhere else, which is every event whose guests somebody
+   * chose one at a time.
+   *
+   * A reminder made out of a bill carries the biller and everyone else the
+   * mail went to. Left on, the primary button reads Send invites, and the
+   * first press mails all of them an invitation to the reader's private note
+   * to self. The switch is right there and says what it does, so inviting them
+   * is one deliberate click; un-sending is not.
+   */
+  const [sendInvites, setSendInvites] = useState(!init.seed?.attendees?.length);
   const [busy, setBusy] = useState(false);
   const [fb, setFb] = useState<Record<string, BusyPeriod[]>>({});
   const [showMore, setShowMore] = useState(Boolean(ev && (ev.privacy !== "public" || ev.freeBusyStatus === "free" || ev.color || ev.status !== "confirmed" || Object.keys(ev.categories ?? {}).length)));