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.
