Add ARIA focus behavior docs and focus tests

[tilucasoli]

Description

Adds comprehensive focus behavior tests for Accordion, Button, and Checkbox. Updates NakedDialog to auto-focus the first focusable element on mount. Refines popover and textfield focus handling for ARIA compliance. Minor UI and code improvements in example and popover components.

Related Issues

#46

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Checklist

Note: Updating the pubspec.yaml and CHANGELOG.md is not required. These are handled automatically during the release process.

• My PR includes unit or integration tests for all changed/updated/fixed behaviors.
• I have updated or added relevant documentation (doc comments with ///).
• I am prepared to follow up on review comments in a timely manner.

Breaking Change

Does this PR require users of the package to manually update their code?

• Yes, this is a breaking change.
• No, this is not a breaking change.

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[github-actions]

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[leoafarias]

Closing — the library changes are superseded, and one would now regress. The tests are still worth having.

The 3 library files (+44/-30) no longer apply:

• naked_dialog.dart — already done on main, and more idiomatically. Main focuses the first focusable descendant via FocusTraversalGroup.maybeOf(...) with a fallback; this PR used SchedulerBinding.addPostFrameCallback plus a manual walk.
• naked_popover.dart — this one is now harmful. It replaces the focus wiring with a bare FocusScope, deleting the Shortcuts/Actions block that supplies dismiss and next/previous-focus intents. Correct in January, a regression against current main.
• naked_textfield.dart — +6/-8, needs re-checking against the same drift.

The 11 focus test files (~5,200 lines) are still missing. There is no test/focus/ on main — only semantics/, src/, mixins/, and utilities/. Coverage for accordion, button, checkbox, dialog, menu, popover, radio, select, slider, tabs, and textfield focus behaviour has no equivalent today.

Suggested salvage: take the test cases, drop the library diff. They need updating to current APIs rather than re-deriving — though the dialog cases were written against this PR's implementation and will need adjusting to main's FocusTraversalGroup approach. That work belongs with #46.

Thanks for the coverage; it is the part that outlived the branch.