feat: Split Metrics tab into sub-tabs and fix early mode storage (v0.4.3)

- Add sub-tabs: Endpoint, Selection, Runtime, Advanced
- Fix early mode checkbox not saving (was outside form element)
- Add CSS styling for horizontal sub-tab navigation
- Update translations with new sub-tab strings

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

wp-prometheus.php

@@ -130,7 +130,7 @@ wp_prometheus_early_metrics_check();
  *
  * @var string
  */
-define( 'WP_PROMETHEUS_VERSION', '0.4.2' );
+define( 'WP_PROMETHEUS_VERSION', '0.4.3' );
 
 /**
  * Plugin file path.