fix: Separate settings groups to prevent cross-tab overwrites (v0.4.5)

Split Metrics sub-tab settings into separate WordPress option groups:
- wp_prometheus_endpoint_settings for auth token
- wp_prometheus_selection_settings for enabled metrics
- wp_prometheus_advanced_settings for isolated mode

This fixes the bug where saving from one sub-tab would clear settings
from other sub-tabs due to all settings sharing a single option group.

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

assets/css/admin.css

@@ -16,7 +16,7 @@
 
 .wp-prometheus-subtab-nav {
     display: flex;
-    margin: 0 0 20px 0;
+    margin: 0;
     padding: 0;
     list-style: none;
     border-bottom: 1px solid #c3c4c7;