Extracts a shared EmailThreadTile widget that handles the Dismissible
swipe (archive/delete), selection-mode checkbox, and account-name
subtitle — eliminating ~150 lines of duplicated rendering code between
EmailListScreen and CombinedInboxScreen.
Also adds multi-select and batch archive/delete to CombinedInboxScreen
(long-press to start, select-all in AppBar, action bar at bottom).
Cross-account batch archive groups threads by accountId to resolve the
right archive folder per account.
SearchScreen is unchanged — it serves a different purpose (cross-entity
discovery) and already uses the shared EmailTile for individual emails.
Closes#424
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
