Feature: recipient autocomplete from Sent, with on-demand server search

Compose recipient fields only suggested existing contacts and directory
users, so people you had emailed before but never saved as a contact
never came up. This adds an Outlook-Web-style suggestion flow.

On startup the Sent folder is read once (metadata only) to build a cache
of addresses you have written to; those are merged into the autocomplete
after contacts and directory principals, deduped, contacts winning.

When the recipient is not in the cache, the dropdown offers a "search the
server" row that queries the Sent folder on demand. That lookup fetches
only the to/cc fields (no subject, body or attachments) and returns the
matching addresses, deduped.

New strings are added to all 20 locales.

app/(main)/[locale]/page.tsx

@@ -121,7 +121,7 @@ export default function Home() {
   useIdentitySync();
   const trustedSendersAddressBook = useSettingsStore((state) => state.trustedSendersAddressBook);
   const sendDelaySeconds = useSettingsStore((state) => state.sendDelaySeconds);
-  const { loadTrustedSendersBook, trustedSendersLoaded } = useContactStore();
+  const { loadTrustedSendersBook, trustedSendersLoaded, loadRecentRecipients } = useContactStore();
 
   const promptForRescheduleDelayedUntil = useCallback((): string | null => {
     const value = window.prompt(t('email_viewer.reschedule_prompt'));
@@ -342,6 +342,15 @@ export default function Home() {
     refreshCurrentMailbox,
   } = useEmailStore();
 
+  // Load recent recipients (from the Sent folder) for compose autocomplete.
+  // Runs once when the Sent mailbox is known; the store guards against reloads.
+  useEffect(() => {
+    const sent = mailboxes.find((m) => m.role === 'sent');
+    if (client && sent) {
+      loadRecentRecipients(client, sent.originalId || sent.id);
+    }
+  }, [client, mailboxes, loadRecentRecipients]);
+
   // Pro shell: populate per-account mailbox cache so the sidebar can render
   // every connected account Thunderbird-style.
   useProMultiAccountMailboxes();