Add admin account page for PHP password management

Implements the admin user management UI as a self-contained PHP password
change/set flow — no SSH or sudo required.

- public/admin/account.php: shows auth status (PHP hash present, credentials
  file path), password change form (requires current password when one exists,
  min 12 chars, confirm field), and a danger-zone form to delete the
  credentials file entirely
- public/admin/actions/account.php: CSRF-guarded POST handler; verifies
  current password via AdminAuth::login() before accepting a new one;
  generates bcrypt (cost 12) hash; writes config/admin_credentials.php
  atomically via a temp file + rename; regenerates session on success;
  redirects to /admin/login.php when credentials are deleted
- templates/admin/head.php: 'Compte' nav link added (active on account.php)
- public/assets/admin.css: .admin-account-status, .admin-section-title,
  .admin-field-hint, .admin-danger-zone component styles added

Note: the nginx htpasswd flow (manage-admin-users.sh) requires root on the
server and is intentionally kept as a CLI-only operation.

templates/admin/head.php

@@ -31,6 +31,7 @@
         <a href="/admin/tags.php" class="admin-nav__link <?= $currentPage === 'tags.php' ? 'active' : '' ?>">Mots-clés</a>
         <a href="/admin/status.php" class="admin-nav__link <?= $currentPage === 'status.php' ? 'active' : '' ?>">Statut</a>
         <a href="/admin/logs.php" class="admin-nav__link <?= $currentPage === 'logs.php' ? 'active' : '' ?>">Journaux</a>
+        <a href="/admin/account.php" class="admin-nav__link <?= $currentPage === 'account.php' ? 'active' : '' ?>">Compte</a>
         <?php if ($thesisId && in_array($currentPage, ['edit.php', 'thanks.php'])): ?>
             <a href="/admin/edit.php?id=<?= intval($thesisId) ?>" class="admin-nav__link <?= $currentPage === 'edit.php' ? 'active' : '' ?>">Modifier</a>
         <?php endif; ?>