Add website fields to project meta box

Adds a 'This project is a website' toggle to the Repository meta box.
When checked, site fields (URL, platform, optional screenshot) replace the
git-repo fields; when unchecked, repo fields are shown and site fields
are cleared. Existing posts default to repo mode.

- admin/metabox.php: checkbox, site URL/platform/screenshot inputs with
  media-library picker, JS toggle, save handler that switches modes
- includes/helper-functions.php: projects_portfolio_is_website / get
  _site_url / get_site_platform / get_site_screenshot_url helpers
- templates/single-projects.php: website-mode rendering (Visit Site
  button, platform row, screenshot)
- tests/test-metabox.php: website-mode persistence + mode-switch tests
- tests/wp-stubs.php: sanitize_text_field, absint stubs

tests/wp-stubs.php

@@ -80,6 +80,12 @@ if ( ! function_exists( 'wp_unslash' ) ) {
 if ( ! function_exists( 'esc_url_raw' ) ) {
     function esc_url_raw( $url ) { return $url; }
 }
+if ( ! function_exists( 'sanitize_text_field' ) ) {
+    function sanitize_text_field( $str ) { return is_string( $str ) ? trim( $str ) : ''; }
+}
+if ( ! function_exists( 'absint' ) ) {
+    function absint( $maybeint ) { return abs( (int) $maybeint ); }
+}
 
 // Minimal WP_Error stub if Brain\Monkey doesn't supply one.
 if ( ! class_exists( 'WP_Error' ) ) {