Move focus deliberately when content changes

Keyboard and screen reader users follow focus. When content changes and focus does not, they lose their place.

• Dialogs: when one opens, focus moves into it and stays there until it closes, then returns to the button that opened it.
• Client-side navigation: move focus to the new page heading or main region so users know the page changed.
• Custom widgets: use a ref and call focus() in the event handler that opens them.