Redesign settings page for v2.0.0 #33
Description
Modernize the settings page UI and add Surf ecosystem context.
- Show config source indicator (PHP filter / .surf-links.yml / database)
- If config comes from filter or YAML, show read-only view with source path
- Clean up settings layout for modern WP admin
- Add link to Surf ecosystem documentation
- Export/import: support `.surf-links.yml` format in addition to serialized PHP
- Settings migration from v1.x option names