* refactor(web): store backups in XDG config directory
Move file backups from `.backups/` alongside the file to
`~/.config/compose-farm/backups/` (respecting XDG_CONFIG_HOME).
The original file path is mirrored inside to avoid name collisions.
* docs(web): document automatic backup location
* refactor(paths): extract shared config_dir() function
* fix(web): use path anchor for Windows compatibility
- Add Web UI page to navigation in zensical.toml
- Use absolute paths for video assets in web-ui.md
- Add web-workflow demo video to homepage Quick Demo section
