[Settings] Replace large PNG from Command Palette page with dynamic background generation

[daverayment]

Summary of the Pull Request

This PR includes code to separately create gradient fills and noise textures. Together, these are used to replace the textured gradient background for the Command Palette settings page, which saves shipping a 2.15 MB PNG.

PR Checklist

• Closes: #xxx

• Communication: I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected
• 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 for new binaries
  • WXS for installer for new binaries and localization folder
  • YML for CI pipeline for new test projects
  • YML for signed pipeline
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

A new BackgroundTextureHelper class provides helpers to create:

1. 'CornerGradients': These are 2x2 pixel ImageSources where you define each corner colour. When assigned to an Image and stretched, the GPU will produce a gradient fill.
2. Noise texture as ImageSource: This returns a grayscale bitmap ImageSource of the desired dimensions. An intensity parameter can control the strength of the noise. Combine this with opacity settings on the receiver control to dial in the desired effect.
3. Noise texture as ImageBrush: Creates a noise texture as an ImageBrush.

For the Command Palette background, which is at most 1000x560 pixels, this takes a few milliseconds on my slow laptop, so the texture is deliberately not cached.

The Command Palette settings page's code behind assigns the background gradient and texture in a Loaded handler, using the existing Image element for the gradient and a new Rectangle for the texture.

Validation Steps Performed

1. Visual comparison:

(Before on top, after beneath.)

2. Manual testing in Debug and Release builds to confirm that navigation was unaffected and the ImageSources were correctly GC'ed after navigating away from the page.

[jiripolasek]

/azp run

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[github-actions]

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... in a clone of the git@github.com:daverayment/PowerToys.git repository
on the settings-optimizecmdpalbackground branch (ℹ️ how do I use this?):

curl -s -S -L 'https://raw.githubusercontent.com/check-spelling/check-spelling/c635c2f3f714eec2fcf27b643a1919b9a811ef2e/apply.pl' |
perl - 'https://github.com/microsoft/PowerToys/actions/runs/19790290644/attempts/1' &&
git commit -m 'Update check-spelling metadata'

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[jiripolasek]

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[niels9001]

@daverayment Oh, just seeing this PR now - this looks really useful for the Windows Community Toolkit, actually? Have you considered adding it?

The design team gave us a gradient that is easier to replicate in XAML, so we have replaced the png with a gradient brush in: #44721. Therefore, closing this PR for now.