fix(ui): improve BackTop accessibility and click handling

[wyf027]

Summary

• add a default aria-label for BackTop so the arrow glyph is not the accessible name
• preserve caller onClick handlers while keeping the built-in scroll-to-top behavior
• allow callers to prevent the built-in scroll with event.preventDefault()
• add BackTop tests and align docs with the actual visibilityHeight default

Validation

• GitHub Actions CI / quality passed for head aa9aac43b482a6d0621ffe807ee1710e1e23f624 in run 28882494903.
• Passed steps: npm run lint:ui, npm run typecheck:ui, npm run test:ui, npm run build:ui, npm run build:docs.
• Test run included new src/components/navigation/BackTop/BackTop.test.tsx coverage for visibility threshold, accessible labelling, click preservation, and preventDefault().
• Not run locally: the Codex shell executor is currently failing with Failed to create unified exec process: No such file or directory.

Notes

• codebase-memory-mcp graph tools were not exposed in this session, so this run used repository files through GitHub as the AGENTS.md fallback.
• This branch starts from current main at 41f28b8caf69b827972396c0a7450ff195373070 and avoids the still-open FloatButton PR fix(ui): label float button from tooltip #86.

[ecc-tools]

ECC bundle files are already tracked in this repository. Skipping generation of another bundle PR.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Wrap the scroll event in act

In this jsdom test environment, dispatching the native scroll event here is outside React Testing Library's act, so the setVisible update from BackTop's window listener is not flushed before the immediate role queries. I verified npm run test:ui -- BackTop: the body remains <div /> and all four new tests fail; wrap this dispatch/call sites in act(...) or wait for the button before asserting.

Useful? React with 👍 / 👎.

[wyf027]

Fixed in aa9aac43: setScrollY now wraps the scroll dispatch in Testing Library act(...). The follow-up CI / quality run 28882494903 passed, including Test UI.

[ecc-tools]

ECC bundle files are already tracked in this repository. Skipping generation of another bundle PR.