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>

CLAUDE.md

@@ -291,6 +291,20 @@ add_action( 'wp_prometheus_collect_metrics', function( $collector ) {
 
 ## Session History
 
+### 2026-02-02 - Sub-tabs & Early Mode Fix (v0.4.3)
+
+- Split Metrics tab into sub-tabs for better organization:
+  - **Endpoint**: Authentication token configuration
+  - **Selection**: Enable/disable individual metrics
+  - **Runtime**: Reset runtime metrics data
+  - **Advanced**: Early mode toggle and status
+- Fixed early mode setting not being saved (was outside form element)
+- Added CSS styling for horizontal sub-tab navigation
+- **Key Learning**: WordPress Settings API form structure
+  - Settings must be inside `<form action="options.php">` with `settings_fields()` call
+  - Each sub-tab needs its own form wrapper for proper saving
+  - Sub-tabs use URL query parameter (`subtab`) within the main tab
+
 ### 2026-02-02 - Early Mode Toggle (v0.4.2)
 
 - Added option to disable early mode for users who need extensibility