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/pages/plugin-config.php

@@ -15,14 +15,14 @@
 
     <?php if (!empty($pluginConfig)): ?>
         <div class="card-footer text-end">
-            <a href="admin.php?route=plugins" class="btn btn-secondary">Annuleren</a>
+            <a href="/admin/plugins" class="btn btn-secondary">Annuleren</a>
             <button type="submit" class="btn btn-primary"><i class="bi bi-check-lg"></i> Opslaan</button>
         </div>
     <?php endif; ?>
 </form>
 
 <div class="mt-3">
-    <a href="admin.php?route=plugins" class="btn btn-outline-secondary btn-sm">
+    <a href="/admin/plugins" class="btn btn-outline-secondary btn-sm">
         <i class="bi bi-arrow-left"></i> Terug naar plugins
     </a>
 </div>