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/account.php

@@ -19,8 +19,8 @@ if (empty($_SESSION['csrf_token'])) {
 <?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">Compte administrateur</h1>
+<main id="main-content">
+    <h1>Compte administrateur</h1>
 
     <?php if ($error): ?>
     <div class="admin-alert admin-alert--error">⚠ <?= htmlspecialchars($error) ?></div>

public/admin/add.php

@@ -42,8 +42,8 @@ function wasSelected($key, $value) {
 <?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">Ajouter un TFE</h1>
+<main id="main-content">
+    <h1>Ajouter un TFE</h1>
 
     <?php if ($error): ?>
     <div class="admin-alert admin-alert--error">⚠ <?= htmlspecialchars($error) ?></div>
@@ -81,8 +81,8 @@ function wasSelected($key, $value) {
         </div>
 
         <!-- Composition du jury -->
-        <fieldset class="admin-fieldset">
-            <legend class="admin-fieldset-legend">Composition du jury</legend>
+        <fieldset>
+            <legend>Composition du jury</legend>
 
             <!-- Président·e -->
             <div class="admin-form-row">

public/admin/edit.php

@@ -64,8 +64,8 @@ try {
 <?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">Modifier un TFE</h1>
+<main id="main-content">
+    <h1>Modifier un TFE</h1>
 
     <?php if ($error): ?>
     <div class="admin-alert admin-alert--error">⚠ <?= htmlspecialchars($error) ?></div>
@@ -148,8 +148,8 @@ try {
                 }
             }
         ?>
-        <fieldset class="admin-fieldset">
-            <legend class="admin-fieldset-legend">Composition du jury</legend>
+        <fieldset>
+            <legend>Composition du jury</legend>
 
             <div class="admin-form-row">
                 <label class="admin-label" for="jury_president">Président·e :</label>

public/admin/import.php

@@ -320,8 +320,8 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_FILES['csv_file'])) {
 <?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">Importer une liste de TFE</h1>
+<main id="main-content">
+    <h1>Importer une liste de TFE</h1>
 
     <?php if (!empty($errors)): ?>
     <div class="admin-alert admin-alert--error">

public/admin/index.php

@@ -64,8 +64,8 @@ document.addEventListener('DOMContentLoaded', () => {
 });
 </script>
 
-<main class="admin-main" id="main-content">
-    <h1 class="admin-page-title">Liste des TFE</h1>
+<main id="main-content">
+    <h1>Liste des TFE</h1>
 
     <?php if (isset($_SESSION['error'])): ?>
     <div class="admin-alert admin-alert--error">⚠ <?= htmlspecialchars($_SESSION['error']); unset($_SESSION['error']); ?></div>
@@ -157,7 +157,7 @@ document.addEventListener('DOMContentLoaded', () => {
     <?php if (empty($theses)): ?>
         <p style="color:var(--admin-text-muted);padding:1rem 0;">Aucun TFE trouvé.</p>
     <?php else: ?>
-    <table class="admin-table">
+    <table>
         <thead>
             <tr>
                 <th scope="col"><input type="checkbox" onchange="toggleAll(this)"></th>

public/admin/pages-edit.php

@@ -81,8 +81,8 @@ JS;
 <?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">Éditer : <?= htmlspecialchars($page['title']) ?></h1>
+<main id="main-content">
+    <h1>Éditer : <?= htmlspecialchars($page['title']) ?></h1>
 
     <form action="/admin/actions/page.php" method="post" class="admin-form">
         <input type="hidden" name="csrf_token" value="<?= htmlspecialchars($_SESSION['csrf_token']) ?>">

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>

public/admin/system.php

@@ -372,8 +372,8 @@ require_once APP_ROOT . '/templates/head.php';
 
 
 
-<main class="admin-main" id="main-content">
-    <h1 class="admin-page-title">Système</h1>
+<main id="main-content">
+    <h1>Système</h1>
 
     <p class="sys-refresh-note">
         Affiché le <?= date('d/m/Y à H:i:s') ?> —

public/admin/tags.php

@@ -25,8 +25,8 @@ unset($_SESSION['admin_error'], $_SESSION['admin_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">Mots-clés (<?= count($tags) ?>)</h1>
+<main id="main-content">
+    <h1>Mots-clés (<?= count($tags) ?>)</h1>
 
     <?php if ($error): ?>
     <div class="admin-alert admin-alert--error">⚠ <?= htmlspecialchars($error) ?></div>
@@ -35,7 +35,7 @@ unset($_SESSION['admin_error'], $_SESSION['admin_success']);
     <div class="admin-alert admin-alert--success">✓ <?= htmlspecialchars($success) ?></div>
     <?php endif; ?>
 
-    <table class="admin-table" style="margin-top:1.5rem;">
+    <table>
         <thead>
             <tr>
                 <th scope="col" style="width:40%;">Nom</th>

public/admin/thanks.php

@@ -65,8 +65,8 @@ $pageTitle = "Récapitulatif TFE";
 <?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">Récapitulatif TFE</h1>
+<main id="main-content">
+    <h1>Récapitulatif TFE</h1>
 
     <?php if ($error): ?>
         <div class="admin-alert admin-alert--error">⚠ <?= htmlspecialchars($error) ?></div>
@@ -122,7 +122,7 @@ $pageTitle = "Récapitulatif TFE";
         <?php if (!empty($files)): ?>
         <section>
             <h2>Fichiers</h2>
-            <table class="admin-table">
+            <table>
                 <thead><tr><th scope="col">Type</th><th scope="col">Fichier</th><th scope="col">Taille</th><th scope="col">Date</th></tr></thead>
                 <tbody>
                     <?php foreach ($files as $f): ?>