Fixes display of author avatar and league links

Ensures author avatar is only displayed if it exists.

Uses dynamic league IDs from the `spielbetrieb` page in the navbar, which
allows changing the league links in the backend.

site/templates/blog.php

@@ -39,7 +39,9 @@
             <?php if ($user = $post->author()->toUser()): ?>
             <div class="mt-6 flex border-t border-gray-900/5 pt-6">
                 <div class="relative flex items-center gap-x-4">
-                    <img src="<?= $user->avatar()->url() ?>" alt="" class="size-10 rounded-full bg-gray-50" />
+                    <?php if ($user->avatar()): ?>
+                        <img src="<?= $user->avatar()->url() ?>" alt="" class="size-10 rounded-full bg-gray-50" />
+                    <?php endif; ?>
                     <div class="text-sm/6">
                         <p>
                             <span class="absolute inset-0"></span>