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.

lib/jmap/client-interface.ts

@@ -89,6 +89,13 @@ export interface IJMAPClient {
     limit?: number,
     position?: number,
   ): Promise<{ emails: Email[]; hasMore: boolean; total: number }>;
+  /**
+   * Lean recipient search for compose autocomplete ("search the server" action):
+   * finds messages in `sentMailboxId` whose to/cc matches `query` and returns
+   * only the matching addresses (fetches just the `to`/`cc` properties - no
+   * bodies or attachments), deduped.
+   */
+  searchSentRecipients(query: string, sentMailboxId: string, accountId?: string, limit?: number): Promise<Array<{ name: string; email: string }>>;
 
   // ── Email mutations ───────────────────────────────────────────
   markAsRead(emailId: string, read?: boolean, accountId?: string): Promise<void>;