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

admin-console/storage/logs/login_attempts.json

@@ -0,0 +1 @@
+{"admi":{"count":1,"last_attempt":1771257322}}