admin.css: replace .admin-main, .admin-page-title, .admin-table, .admin-fieldset with semantic selectors

Replace four presentational class names in admin.css with structural selectors
that target native HTML elements already present in every admin template:

  .admin-main           → .admin-body main
  .admin-page-title     → .admin-body main > h1
  .admin-table          → .admin-body table
  .admin-fieldset       → .admin-body fieldset
  .admin-fieldset-legend → .admin-body legend

Also migrate the .admin-main > section / h2 / dl / dt / dd block to
.admin-body main > section so the thanks-page section styles survive.

Add .admin-body main > table { margin-top: 1.5rem } to absorb the inline
style="margin-top:1.5rem" that was on tags.php's <table class="admin-table">.

All 10 affected admin templates updated (add, edit, account, index, import,
pages, pages-edit, tags, system, thanks) — class attributes removed where
the element alone is now the selector.  Zero visual changes.

public/admin/pages.php

@@ -21,14 +21,14 @@ unset($_SESSION['success']);
 <?php $isAdmin = true; $bodyClass = 'admin-body'; require_once APP_ROOT . '/templates/head.php'; ?>
     <?php include APP_ROOT . '/templates/header.php'; ?>
 
-<main class="admin-main" id="main-content">
-    <h1 class="admin-page-title">Pages statiques</h1>
+<main id="main-content">
+    <h1>Pages statiques</h1>
 
     <?php if ($success): ?>
     <div class="admin-alert admin-alert--success">✓ <?= htmlspecialchars($success) ?></div>
     <?php endif; ?>
 
-    <table class="admin-table">
+    <table>
         <thead>
             <tr>
                 <th scope="col">Slug</th>