Redesign settings page for v2.0.0

[apermo]

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