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.

locales/ko/common.json

@@ -677,7 +677,9 @@
     "recipient_edit_email": "이메일 주소 편집",
     "recipient_edit_name": "표시 이름 편집",
     "recipient_email_placeholder": "이메일 주소",
-    "recipient_name_placeholder": "표시 이름"
+    "recipient_name_placeholder": "표시 이름",
+    "autocomplete_search_server": "서버에서 검색",
+    "autocomplete_searching": "검색 중..."
   },
   "confirm_dialog": {
     "confirm": "확인",