fix(12.1-01): restore asyncio import, bump version to 0.2.3, update docs

- Restore asyncio import removed when legacy list_folder was deleted (health_check
  still uses asyncio.to_thread); fixes test_nextcloud_connect_persists regression
- Bump backend/main.py and frontend/package.json version to 0.2.3
- Update CLAUDE.md: note Phase 12.1 Plan 01 complete, add normalize_nextcloud_url
  to shared module map, add NextcloudBackend no-override rule
- Update README.md: version 0.2.3, note canonical Nextcloud URL normalization and
  OneDrive nextLink origin guard
- Full backend suite: 585 passed, 1 pre-existing failure (missing python-docx module)

backend/storage/nextcloud_backend.py

@@ -29,6 +29,8 @@ Credentials dict shape (same as WebDAVBackend):
 """
 from __future__ import annotations
 
+import asyncio
+
 from storage.cloud_utils import validate_cloud_url
 from storage.webdav_backend import WebDAVBackend