feat(public): add About Us page

Adds a public "درباره ما" page at /درباره-ما reusing the landing theme
(same webpack entry, header, footer, registration modal). Section ids
match the header anchors so the nav works on this page too.

The explicit route takes precedence over LandingController's catch-all
/{slug}, which is registered with priority -10.

Also links it from the header nav and footer quick links, and lists it
in sitemap.xml.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/Shared/Controller/SeoController.php

@@ -36,7 +36,8 @@ class SeoController extends AbstractController
 
         $xml = '<?xml version="1.0" encoding="UTF-8"?>' . "\n"
             . '<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">' . "\n"
-            . $this->urlNode("{$base}/", 'weekly', '1.0');
+            . $this->urlNode("{$base}/", 'weekly', '1.0')
+            . $this->urlNode($base . '/' . rawurlencode(AboutController::SLUG), 'yearly', '0.5');
 
         // اسلاگ‌ها فارسی‌اند و باید percent-encode شوند، وگرنه XML نامعتبر است.
         foreach ($this->landings->all() as $page) {