Add Tab/Shift+Tab focus trap to the dialog element:
- onKeyDown handler queries all focusable elements inside dialogRef
- Tab from last element wraps to first (preventDefault)
- Shift+Tab from first element wraps to last (preventDefault)
- No new dependency — implemented inline with dialogRef
- Existing focus-on-open (titleRef) and Escape-to-close unchanged
- Update docblock: focus trap claim is now accurate (WR-07)