Set tabindex=-1 on hidden column headers for keyboard accessibility

Problem/Motivation

In DataTables responsive mode, column headers that are collapsed/hidden remain in the DOM but are visually hidden. However, their sort controls are still keyboard-focusable via Tab key.

Keyboard users can focus on invisible sort controls, which violates WCAG 2.4.3 (Focus Order) and WCAG 2.4.7 (Focus Visible) - users should only be able to focus on elements they can see.

Proposed resolution

Set tabindex="-1" on column headers (<th>) that are:

• Collapsed in responsive mode
• Fixed columns that are off-screen
• Otherwise visually hidden

This removes them from the keyboard tab order without affecting mouse interaction or accessibility tree.

Remaining tasks

• [ ] Detect hidden/collapsed column headers
• [ ] Dynamically set/remove tabindex=-1 based on visibility
• [ ] Test keyboard navigation
• [ ] Verify screen reader behavior remains correct