Clean URLs and security improvements

- Add .htaccess rewrite rules for clean URLs (/nl/page, /admin/route)
- Add PHP dev server router with clean URL support
- Update admin template asset paths to absolute for clean URL compat
- All pentest fixes verified: CSRF on login, directory listing disabled,
  secure cookies, backup/sourcemap files removed, version disclosure off

admin/templates/login.php

@@ -4,8 +4,8 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>CodePress Admin - Login</title>
-    <link rel="stylesheet" href="assets/css/bootstrap.min.css">
-    <link rel="stylesheet" href="assets/css/bootstrap-icons.css">
+    <link rel="stylesheet" href="/assets/css/bootstrap.min.css">
+    <link rel="stylesheet" href="/assets/css/bootstrap-icons.css">
     <style>
         body { background-color: #f5f6fa; }
         .login-card { max-width: 400px; margin: 10vh auto; }
@@ -26,7 +26,7 @@
                             <button type="button" class="btn-close" data-bs-dismiss="alert"></button>
                         </div>
                     <?php endif; ?>
-                    <form method="POST" action="admin.php?route=login">
+                    <form method="POST" action="/admin/login">
                         <input type="hidden" name="csrf_token" value="<?= htmlspecialchars($csrfToken) ?>">
                         <div class="mb-3">
                             <label for="username" class="form-label">Gebruikersnaam</label>
@@ -51,10 +51,10 @@
                 </div>
             </div>
             <p class="text-center text-muted mt-3 small">
-                <a href="index.php" class="text-decoration-none"><i class="bi bi-arrow-left"></i> Terug naar website</a>
+                <a href="/" class="text-decoration-none"><i class="bi bi-arrow-left"></i> Terug naar website</a>
             </p>
         </div>
     </div>
-    <script src="assets/js/bootstrap.bundle.min.js"></script>
+    <script src="/assets/js/bootstrap.bundle.min.js"></script>
 </body>
 </html>