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/partials/toast.php

@@ -0,0 +1,12 @@
+<?php if ($flash['error']): ?>
+<p class="toast toast--error" role="alert">
+    <span class="toast__icon" aria-hidden="true">⚠</span>
+    <?= htmlspecialchars($flash['error']) ?>
+</p>
+<?php endif; ?>
+<?php if ($flash['success']): ?>
+<p class="toast toast--success" role="status">
+    <span class="toast__icon" aria-hidden="true">✓</span>
+    <?= htmlspecialchars($flash['success']) ?>
+</p>
+<?php endif; ?>