Initial commit: webmail client (M0/M1)

Three-pane webmail client.
- Backend: FastAPI + async SQLAlchemy + arq worker (Postgres/Redis), IMAP/SMTP
  sync, threading, search, drafts/outbox with undo, attachments, 2FA.
- Frontend: React 18 + Vite + TypeScript + TanStack Query + Tailwind; three-pane
  UI with rich-text composer, light/dark themes, resizable panels.

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

backend/app/api/cursors.py

@@ -0,0 +1,23 @@
+"""Keyset cursor helpers for thread listings (SPEC §9.1: cursor = last_message_at,id)."""
+
+from __future__ import annotations
+
+import base64
+import datetime as dt
+import uuid
+
+
+def encode_cursor(last_message_at: dt.datetime, thread_id: uuid.UUID) -> str:
+    raw = f"{last_message_at.isoformat()}|{thread_id}"
+    return base64.urlsafe_b64encode(raw.encode()).decode()
+
+
+def decode_cursor(cursor: str | None) -> tuple[dt.datetime, uuid.UUID] | None:
+    if not cursor:
+        return None
+    try:
+        raw = base64.urlsafe_b64decode(cursor.encode()).decode()
+        date_str, id_str = raw.split("|", 1)
+        return dt.datetime.fromisoformat(date_str), uuid.UUID(id_str)
+    except (ValueError, TypeError):
+        return None