Add admin console with login, dashboard, content/config/plugin/user management

File-based admin panel accessible at /admin.php with:
- Session-based auth with bcrypt hashing and brute-force protection
- Dashboard with site statistics and quick actions
- Content manager: browse, create, edit, delete files
- Config editor with JSON validation
- Plugin overview with status indicators
- User management: add, remove, change passwords
- CSRF protection on all forms, path traversal prevention
- Updated README (NL/EN) and guides with admin documentation

guide/en.codepress.md

@@ -50,6 +50,17 @@ CodePress is a lightweight, file-based Content Management System built with PHP
 - **Dynamic layouts** with YAML frontmatter
 - **Sidebar support** with plugin integration and toggle function via breadcrumb
 
+### 🛡️ Admin Console
+- Built-in admin panel at `/admin.php`
+- **Dashboard** with statistics and quick actions
+- **Content management** - Browse, create, edit and delete files
+- **Configuration editor** - Edit `config.json` with JSON validation
+- **Plugin overview** - Status of all installed plugins
+- **User management** - Add, remove users and change passwords
+- Session-based authentication with bcrypt hashing
+- CSRF protection, brute-force lockout (5 attempts, 15 min)
+- Default login: `admin` / `admin` (change immediately after installation)
+
 ## Installation
 
 1. Clone or download CodePress files