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/footer.php

@@ -1,5 +1,11 @@
-<!-- Fixed toast container (bottom-center, always visible) -->
-<div id="toast-container" aria-live="polite"></div>
+<!-- Toast region — polled by HTMX after page load -->
+<aside id="toast-region"
+       aria-live="polite"
+       hx-get="/admin/toast-fragment.php"
+       hx-trigger="load"
+       hx-swap="innerHTML"
+       hx-target="#toast-region">
+</aside>
 
 <?php foreach ($extraJs ?? [] as $js): ?>
 <script src="<?= App::assetV($js) ?>"></script>
@@ -8,30 +14,5 @@
 <script><?= $extraJsInline ?></script>
 <?php endif; ?>
 <script src="/assets/js/htmx.min.js"></script>
-<script>
-(function () {
-    var box = document.getElementById('toast-container');
-    if (!box) return;
-
-    // 1) Flash messages from hidden #admin-toasts
-    var src = document.getElementById('admin-toasts');
-    if (src) {
-        src.querySelectorAll('.toast').forEach(function (t) { box.appendChild(t); });
-        src.remove();
-    }
-
-    // 2) Orphaned .toast elements rendered inline by pages (login, thanks, etc.)
-    document.querySelectorAll('.toast:not(#toast-container .toast)')
-        .forEach(function (t) { box.appendChild(t); });
-
-    // Auto-dismiss every toast after 4 seconds
-    box.querySelectorAll('.toast').forEach(function (toast, i) {
-        setTimeout(function () {
-            toast.classList.add('toast-exit');
-            toast.addEventListener('animationend', function () { toast.remove(); });
-        }, 4000 + (i * 200));
-    });
-})();
-</script>
 </body>
 </html>