collapse cross-account dupes, Lato UI font, mail-list selection & density tuning

Backend:
- Collapse identical cross-account deliveries in a thread: new messages.delivery_id
  (migration 0011) preserves the real Message-ID even when message_id_hdr is
  synthesized (§7.1); get_thread groups by it and reports also_in_account_ids; the
  list count counts distinct deliveries. Backfill script for existing rows.

Frontend:
- Bundle Lato (SIL OFL) as the UI font; unread mail-list rows use Black weight.
- Mail-list selection: hover-avatar checkbox, tri-state "select all", bulk
  Archive/Pin/Delete (Restore/Delete in Archive/Trash/Spam) with a count in the
  reading pane; unread blue dot; "also delivered to" note in the thread.
- Spacing/size tuning: sender-column width, icon gaps, avatar size, bar heights,
  and a round of -1px font sizes across the sidebar, mail list, and reading pane.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U72FZiF1W1pb8XxyWBTREA

backend/app/indexer/indexer.py

@@ -161,6 +161,9 @@ async def persist_message(
         mailbox_id=mailbox_id,
         uid=remote.uid,
         message_id_hdr=message_id_hdr,
+        # Keep the real delivered Message-ID (may differ from the stored, possibly
+        # synthesized, message_id_hdr) so cross-account copies collapse in threads.
+        delivery_id=parsed.message_id,
         in_reply_to=parsed.in_reply_to,
         references_hdr=parsed.references,
         subject=parsed.subject,