Add IMAP search: server-side OR SUBJECT/TEXT, inline results in email list

- EmailRepository: add searchEmails(accountId, mailboxPath, query)
- EmailRepositoryImpl: UID SEARCH with OR SUBJECT/TEXT criteria,
  fetch ENVELOPE+FLAGS for matching UIDs
- EmailListScreen: toggle search bar in AppBar; submit triggers server
  search; results replace the stream list; ESC/back closes search
- Refactored list into _buildList() shared by stream and search views
- README/PLAN.md updated

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -151,4 +151,5 @@ test/
 - **Attachment indicators** — paperclip icon in email list; filename + size in detail
 - **Delete email** — removes from server (IMAP expunge) and local DB
 - **Settings** — list and remove accounts
+- **Search** — IMAP server-side search (subject + body); results shown inline, no navigation change
 - **Offline-first** — all reads come from local Drift/SQLite DB; network only for sync and send