feat: Add dashboard extension hook for third-party plugins (v0.4.6)

Add wp_prometheus_register_dashboards action hook allowing third-party
plugins to register their own Grafana dashboard templates.

- DashboardProvider: registration system with file/JSON support
- Security: path traversal protection, JSON validation
- Admin UI: "Extension" badge and plugin attribution
- Isolated mode support for dashboard registration hook

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

wp-prometheus.php

@@ -3,7 +3,7 @@
  * Plugin Name: WP Prometheus
  * Plugin URI: https://src.bundespruefstelle.ch/magdev/wp-prometheus
  * Description: Prometheus metrics endpoint for WordPress with extensible hooks for custom metrics.
- * Version: 0.4.5
+ * Version: 0.4.6
  * Requires at least: 6.4
  * Requires PHP: 8.3
  * Author: Marco Graetsch
@@ -169,7 +169,7 @@ wp_prometheus_early_metrics_check();
  *
  * @var string
  */
-define( 'WP_PROMETHEUS_VERSION', '0.4.5' );
+define( 'WP_PROMETHEUS_VERSION', '0.4.6' );
 
 /**
  * Plugin file path.