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/config/admin.json

@@ -0,0 +1,15 @@
+{
+    "users": [
+        {
+            "username": "admin",
+            "password_hash": "$2y$12$nUpoaCNZZFL8kOTHNC85q.dTy0hWRmPoF3dAn4GcvSXERMioYr5b6",
+            "role": "admin",
+            "created": "2025-01-01"
+        }
+    ],
+    "security": {
+        "session_timeout": 1800,
+        "max_login_attempts": 5,
+        "lockout_duration": 900
+    }
+}