[A11y] Add semantics tree UI (#9982)
tracking issue: https://github.com/flutter/devtools/issues/9893
Added the tree UI
<img width="1341" height="906" alt="Screenshot 2026-08-27 at 16 59 03" src="https://github.com/user-attachments/assets/23cb465d-48c7-4f79-a916-8524f5b3d6a8" />
### General checklist
### Issues checklist
### Tests checklist
### AI-tooling checklist
* [ ] I read the [AI contributions guidelines] and agree to follow them.
* [ ] I reviewed all AI-generated code before opening this PR.
* [ ] I understand and am able to discuss the code in this PR.
* [ ] I have verifed the accuracy of any AI-generated text included in the PR description.
* [ ] I commit to verifying the accuracy of any AI-generated code or text that I upload in response to review comments.
### Feature-change checklist
* [ ] I added the `release-notes-not-required` label or left a comment requesting the label be added.
* [ ] I added an entry to `packages/devtools_app/release_notes/NEXT_RELEASE_NOTES.md`.
* [ ] I included before/after screenshots and/or a GIF demo of the new UI to my PR description.
* [ ] I ran the DevTools app locally to manually verify my changes.
![build.yaml badge]
If you need help, consider asking for help on [Discord].
[`contributions-welcome`]: https://github.com/flutter/devtools/issues?q=state%3Aopen%20label%3Acontributions-welcome
[`good-first-issue`]: https://github.com/flutter/devtools/issues?q=state%3Aopen%20label%3Agood-first-issue
[build.yaml badge]: https://github.com/flutter/devtools/actions/workflows/build.yaml/badge.svg
