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/system-nginx-config-panel.php

@@ -0,0 +1,40 @@
+<?php if ($nginxConfigMeta): ?>
+<div class="log-meta">
+    <span data-label="Fichier"><?= htmlspecialchars($nginxConfigMeta['path']) ?></span>
+    <span data-label="Taille"><?= $nginxConfigMeta['size'] ?></span>
+    <span data-label="Modifié"><?= $nginxConfigMeta['mtime'] ?></span>
+    <?php if ($nginxConfigSource === 'live'): ?>
+    <span class="nginx-source-badge nginx-source-badge--live">● Config déployée</span>
+    <?php else: ?>
+    <span class="nginx-source-badge nginx-source-badge--local">⚠ Référence locale (config live inaccessible)</span>
+    <?php endif; ?>
+</div>
+<?php endif; ?>
+
+<?php if ($nginxConfigError !== null): ?>
+    <div class="log-unavailable">
+        <strong>Configuration nginx non disponible</strong>
+        <div class="log-unavail-path"><?= htmlspecialchars($nginxConfigError) ?></div>
+        <?php if (php_sapi_name() === 'cli-server'): ?>
+        <div class="log-unavail-dev">
+            En développement, <code>/etc/nginx/sites-available/posterg</code> n'existe pas.
+            La config de référence se trouve dans <code>nginx/posterg.conf</code>.
+        </div>
+        <?php endif; ?>
+    </div>
+
+<?php elseif (empty($nginxConfigLines)): ?>
+    <div class="log-empty">Le fichier de configuration est vide.</div>
+
+<?php else: ?>
+    <div class="log-output" id="log-output" role="region" aria-label="Configuration nginx">
+        <button class="log-copy-btn" id="log-copy-btn" type="button" title="Copier la configuration"
+                onclick="copyLogContent(this);return false">
+            Copier
+        </button>
+        <?php foreach ($nginxConfigLines as $i => $line): ?>
+        <span class="log-line <?= SystemController::nginxLineClass($line) ?>"
+              data-n="<?= $i + 1 ?>"><?= htmlspecialchars($line, ENT_QUOTES | ENT_SUBSTITUTE) ?></span>
+        <?php endforeach; ?>
+    </div>
+<?php endif; ?>