CmdPal: Improve readability and a11y of sections (#44556)

## Summary of the Pull Request

This PR improves visuals and a11y of sections:
- Changes section title color from disabled to secondary text fill.
- Removes separator line if the text is present and indents
- Changes gallery grid subtitle color from tertiary to secondary text
fill.

## Pictures? Pictures!

<img width="850" height="718" alt="image"
src="https://github.com/user-attachments/assets/2e0bffa2-045f-48d9-bff5-dcc561395c6a"
/>

<img width="850" height="773" alt="image"
src="https://github.com/user-attachments/assets/69081472-7f4b-489a-b0e5-5778894fef97"
/>

<img width="850" height="773" alt="image"
src="https://github.com/user-attachments/assets/aa3f0e73-def8-45cb-9f9a-6e9d7e0e6137"
/>

<img width="850" height="773" alt="image"
src="https://github.com/user-attachments/assets/cb8f6bdf-9288-4b6e-b8b8-94f8c83e3ffc"
/>



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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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## Validation Steps Performed

src/modules/cmdpal/ext/SamplePagesExtension/Pages/SectionsPages/SectionsIndexPage.cs

@@ -35,6 +35,10 @@ internal sealed partial class SectionsIndexPage : ListPage
                {
                    Title = "A Gallery grid page with sections",
                },
+               new ListItem(new SampleListPageWithSections(new GalleryGridLayout() { ShowTitle = false, ShowSubtitle = false }))
+               {
+                   Title = "A Gallery grid page without labels with sections",
+               },
             ];
     }
 }