Build master #9

feat(backup): add browser-side WebDAV JSON backup feature

- introduce dedicated WebDavBackupPage component
- implement configuration UI for WebDAV endpoint, credentials, directory and filename prefix
- support automatic directory creation on target WebDAV server
- export all (or selected) auth JSON files into single timestamped snapshot
- store backup config in localStorage (password optionally persisted)
- use direct PUT upload from browser with Basic Auth
- add progress feedback and basic last-backup status display

Also updates README with new feature description and adds navigation entry in the main interface.

This commit introduces a new major frontend feature → `feat` is appropriate.
Primary scope is the new backup functionality → `backup` is a clear scope choice.