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>

.env.example

@@ -0,0 +1,13 @@
+APP_DOMAIN=dev.webmail.local
+DATABASE_URL=postgresql+asyncpg://webmail:webmail_dev@127.0.0.1:5433/webmail_dev
+REDIS_URL=redis://127.0.0.1:6380/0
+SEARCH_BACKEND=pg                    # switch to meili in M2 work
+MEILI_URL=http://127.0.0.1:7700
+MEILI_KEY=dev_master_key_change_me
+MASTER_KEY=CHANGE_ME_base64_32_bytes   # generate: openssl rand -base64 32
+BLOB_ROOT=./var/blobs
+UNDO_SECONDS=10
+POLL_INTERVAL=120
+SESSION_TTL_DAYS=30
+MAX_ATTACHMENT_MB=25
+LOG_LEVEL=DEBUG