Add server-side IMAP search

Search previously only filtered the cached inbox. Now a query also runs an IMAP SEARCH
on the server and folds the matches into the cache, so messages beyond the synced
window surface in the results.

- ImapClient.search(query) ORs SUBJECT/FROM/BODY terms and fetches matching headers
  (extracted a shared toFetchedMessage mapper, reused by fetchRecentInbox).
- MailRepository.searchServer inserts/updates matches into the message cache (no
  pruning); MailboxViewModel triggers it from a debounced, deduplicated search query.
- assemble/test/lint green, including a new ImapClient test asserting SEARCH returns
  only the matching message against GreenMail.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -18,7 +18,7 @@ experience with power-user features tucked under an **Advanced Settings** group.
 > foreground **IMAP IDLE** service; **attachments** — downloaded on demand and opened in a
 > system viewer, and attach files when composing; **multiple accounts** — a unified inbox
 > with per-account filtering; and
-> **search** across cached mail. Outlook Graph send is upcoming.
+> **search** across cached mail and the server (IMAP SEARCH). Outlook Graph send is upcoming.
 
 ## Features (target MVP)