Fix stale tablesort.$th reference after header clicks

[dusan-maintains]

Summary

• Update tablesort.$th on every successful sort call so the instance always tracks the currently sorted header.
• Keep the minified artifact in sync with the source file.

Why

• tablesort.$th is documented as "the <th> currently sorted by", but it stays stale/null after click sorting.
• This directly addresses open reports:
  • Fixes Currently sorted column not updated #40
  • Fixes Memorize current sorted column when sorting by click #45

Test Plan

1. Initialize plugin: $('table').tablesort().
2. Click one header, then another.
3. After each click, verify $('table').data('tablesort').$th points to the clicked <th>.
4. Confirm existing sort direction toggling behavior is unchanged.

Compatibility

• No API changes.
• Existing sorting behavior is preserved; this only keeps documented instance state accurate.