Add ItemsRepeater focus restoration on Extensions settings page (part deux) (#45903)

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## Summary of the Pull Request

Fixes focus management in the Command Palette Extensions settings page.
After user moves through the extension list, when using Shift or
Shift+Tab to navigate into the extensions list, focus now properly
returns to the previously selected extension card instead of jumping to
the first item or end of the list.

This is part of the a11y bug batch.
User Impact:
Keyboard-only and assistive-technology users may lose context and
experience confusion due to unexpected focus movement, increasing
navigation effort and reducing usability of the Extensions page.

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## PR Checklist

- [ ] Closes: #xxx
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- [x] **Communication:** I've discussed this with core contributors
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- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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https://github.com/user-attachments/assets/2ebe25e4-015d-4804-8ae9-9a0107f39b8e

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Co-authored-by: Jiří Polášek <[email protected]>