Self-serve listing validation; changelog pages render server-side

Added

• Self-serve listing on /submit — enter your domain to validate your releases.json manifest and activate listing immediately; valid manifests show a plain-language preview of detected release locations with an activate button for unlisted domains; invalid manifests display field-level errors with a re-check loop. New POST /v1/listing/validate and POST /v1/listing/activate endpoints support the same flow for API and agent callers.

Changed

• Changelog pages load faster — /changelog routes now render markdown server-side; shiki and react-markdown no longer ship to the browser on these pages.