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-log-panel.php

@@ -0,0 +1,54 @@
+<div class="log-toolbar">
+    <form id="lines-form" hx-get="/admin/system-fragment.php"
+          hx-target="#sys-tab-panel"
+          hx-swap="innerHTML"
+          hx-indicator="#sys-tab-panel"
+          hx-trigger="change"
+          hx-vals='{"tab":"<?= htmlspecialchars($activeTab) ?>"}'>
+      <label for="lines-select">Afficher</label>
+      <select id="lines-select" name="n" aria-label="Nombre de lignes">
+        <?php foreach (SystemController::ALLOWED_LINES as $opt): ?>
+        <option value="<?= $opt ?>" <?= $opt === $selectedN ? 'selected' : '' ?>><?= $opt ?> dernières lignes</option>
+        <?php endforeach; ?>
+      </select>
+    </form>
+    <?php if ($logLines !== null && count($logLines) > 0): ?>
+    <span class="log-count-badge"><?= count($logLines) ?> ligne(s)</span>
+    <?php endif; ?>
+</div>
+
+<?php if ($logFileMeta): ?>
+<div class="log-meta">
+    <span data-label="Fichier"><?= htmlspecialchars(SystemController::LOG_FILES[$activeTab]['path']) ?></span>
+    <span data-label="Taille"><?= $logFileMeta['size'] ?></span>
+    <span data-label="Modifié"><?= $logFileMeta['mtime'] ?></span>
+</div>
+<?php endif; ?>
+
+<?php if ($logError !== null): ?>
+    <div class="log-unavailable">
+        <strong>Journaux non disponibles</strong>
+        <div class="log-unavail-path"><?= $logError ?></div>
+        <?php if (php_sapi_name() === 'cli-server'): ?>
+        <div class="log-unavail-dev">
+            En environnement de développement, les logs nginx ne sont pas disponibles.
+            Cette page est pleinement fonctionnelle sur le serveur de production.
+        </div>
+        <?php endif; ?>
+    </div>
+
+<?php elseif (empty($logLines)): ?>
+    <div class="log-empty">Le fichier journal est vide.</div>
+
+<?php else: ?>
+    <div class="log-output" id="log-output" role="log" aria-live="off" aria-label="Contenu du journal">
+        <button class="log-copy-btn" id="log-copy-btn" type="button" title="Copier le contenu"
+                onclick="copyLogContent(this);return false">
+            Copier
+        </button>
+        <?php foreach ($logLines as $i => $line): ?>
+        <span class="log-line <?= SystemController::logLineClass($line) ?>"
+              data-n="<?= count($logLines) - $i ?>"><?= htmlspecialchars($line, ENT_QUOTES | ENT_SUBSTITUTE) ?></span>
+        <?php endforeach; ?>
+    </div>
+<?php endif; ?>