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/scripts/backfill_delivery_id.py

@@ -0,0 +1,35 @@
+"""One-off: recover delivery_id for messages whose stored Message-ID was
+synthesized (§7.1 cross-account duplicate). The real Message-ID is still in the
+raw blob, so re-parse it. Non-synthetic rows were already backfilled by migration
+0011; here we only touch rows left with delivery_id IS NULL that have a real
+Message-ID in their raw bytes."""
+
+import asyncio
+
+from sqlalchemy import select
+
+from app.db import session_scope
+from app.indexer import blobstore
+from app.indexer.mime import parse_message
+from app.models import Message
+
+
+async def main() -> None:
+    async with session_scope() as session:
+        rows = list(
+            await session.scalars(select(Message).where(Message.delivery_id.is_(None)))
+        )
+        fixed = 0
+        for m in rows:
+            try:
+                raw = blobstore.load(m.user_id, m.blob_sha256)
+            except FileNotFoundError:
+                continue
+            parsed = parse_message(raw)
+            if parsed.message_id:
+                m.delivery_id = parsed.message_id
+                fixed += 1
+        print(f"scanned {len(rows)} null rows, set delivery_id on {fixed}")
+
+
+asyncio.run(main())