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

includes/helper-functions.php

@@ -104,6 +104,56 @@ function projects_portfolio_get_repo_browse_url( int $post_id ): string {
     return projects_portfolio_get_provider( $post_id )->get_repo_browse_url();
 }
 
+/**
+ * Whether this project is configured as a website rather than a git repo.
+ *
+ * @since  1.4.0
+ * @param  int    $post_id
+ * @return bool
+ */
+function projects_portfolio_is_website( int $post_id ): bool {
+    return (bool) get_post_meta( $post_id, '_projects_portfolio_is_website', true );
+}
+
+/**
+ * Public URL for the project's website (when configured as a website).
+ *
+ * @since  1.4.0
+ * @param  int    $post_id
+ * @return string
+ */
+function projects_portfolio_get_site_url( int $post_id ): string {
+    return (string) get_post_meta( $post_id, '_projects_portfolio_site_url', true );
+}
+
+/**
+ * Platform label for the project's website (when configured as a website).
+ *
+ * @since  1.4.0
+ * @param  int    $post_id
+ * @return string
+ */
+function projects_portfolio_get_site_platform( int $post_id ): string {
+    return (string) get_post_meta( $post_id, '_projects_portfolio_site_platform', true );
+}
+
+/**
+ * Screenshot attachment URL for the project's website.
+ *
+ * @since  1.4.0
+ * @param  int    $post_id
+ * @param  string $size
+ * @return string
+ */
+function projects_portfolio_get_site_screenshot_url( int $post_id, string $size = 'large' ): string {
+    $attachment_id = (int) get_post_meta( $post_id, '_projects_portfolio_site_screenshot_id', true );
+    if ( $attachment_id <= 0 ) {
+        return '';
+    }
+    $src = wp_get_attachment_image_src( $attachment_id, $size );
+    return $src[0] ?? '';
+}
+
 /**
  * GitHub API call for owner data
  *