Dock: dock visual nits and bits (#45873)

## Summary of the Pull Request

- Improves vertical (left/right) dock layout:
  - Dock items are not clipped;
  - Items with label(s) are stretched to full width;
  - Items without label(s) are centered.
- Top button is in the absolute corner to make clicking to it easier.
- Dock items now have a min width and height 32px.
- Removes a duplicate "No background" label in the expander body on the
Dock settings page.

## Pictures? Pictures!
Vertical layout:
<img width="175" height="1439" alt="image"
src="https://github.com/user-attachments/assets/4f11d9e9-405f-4d9a-8dee-4c8912b88c7c"
/>

Horizontal layout:
<img width="5119" height="49" alt="image"
src="https://github.com/user-attachments/assets/8420ef26-575e-4427-8db8-f9793e3b3572"
/>



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## PR Checklist

- [ ] Closes: #xxx
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- [ ] **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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