Separate admin views from controllers — move HTML to templates/admin/

All admin pages refactored to thin controllers + pure view templates, mirroring
the public-page pattern:

Controllers (public/admin/*.php): auth, data loading, include template
Views (templates/admin/*.php): pure HTML/PHP output
Fragment partials (templates/admin/partials/): toast, system-log-panel, system-nginx-config-panel

Pages migrated: login, tags, contenus, contenus-edit, account, acces-etudiante,
thanks, add, edit, parametres, system, index

Fragment endpoints refactored: system-fragment.php, toast-fragment.php
Skipped (pure redirects): logout, logs, status, import

app/templates/admin/contenus.php

@@ -0,0 +1,61 @@
+<main id="main-content">
+    <h1>Contenus</h1>
+
+    <h2>Pages statiques</h2>
+
+    <table>
+        <thead>
+            <tr>
+                <th scope="col">Slug</th>
+                <th scope="col">Titre</th>
+                <th scope="col">Mis à jour</th>
+                <th scope="col">Action</th>
+            </tr>
+        </thead>
+        <tbody>
+            <?php foreach ($pages as $p): ?>
+            <tr>
+                <td><code><?= htmlspecialchars($p['slug']) ?></code></td>
+                <td><?= htmlspecialchars($p['title']) ?></td>
+                <td><?= htmlspecialchars($p['updated_at'] ?? '—') ?></td>
+                <td>
+                    <a href="/admin/contenus-edit.php?slug=<?= urlencode($p['slug']) ?>"
+                       class="admin-btn admin-btn--sm">Éditer</a>
+                </td>
+            </tr>
+            <?php endforeach; ?>
+        </tbody>
+    </table>
+
+    <h2 style="margin-top:2rem;">À propos</h2>
+
+    <table>
+        <thead>
+            <tr>
+                <th scope="col">Clé</th>
+                <th scope="col">Type</th>
+                <th scope="col">Mis à jour</th>
+                <th scope="col">Action</th>
+            </tr>
+        </thead>
+        <tbody>
+            <?php foreach ($aproposKeys as $a): ?>
+            <?php
+                $typeLabel = match($a['key']) {
+                    'contacts' => 'Contacts',
+                    'credits'  => 'Crédits',
+                };
+            ?>
+            <tr>
+                <td><code><?= htmlspecialchars($a['key']) ?></code></td>
+                <td><?= htmlspecialchars($typeLabel) ?></td>
+                <td><?= htmlspecialchars($a['updated_at'] ?? '—') ?></td>
+                <td>
+                    <a href="/admin/contenus-edit.php?apropos=<?= urlencode($a['key']) ?>"
+                       class="admin-btn admin-btn--sm">Éditer</a>
+                </td>
+            </tr>
+            <?php endforeach; ?>
+        </tbody>
+    </table>
+</main>