[Enhancement] Add background color customization

Before submitting

• I am using the latest version of Quarkdown.
• I have checked the wiki to confirm this feature doesn't already exist.
• I have searched existing issues for duplicates.

Description

This is a sub-issue of #520. Whereas that issue asks for more complex background controls, it makes sense to kick off with solid background colors, similarly to what .container does. This should ideally support all 4 document types.

Proposed syntax or behavior

.pageformat background:{orange}

.pageformat background:{#FF0000}

On paged documents, allow local configurations (should be straightforward by architecture):

.pageformat pages:{1..2} background:{orange}

Additional context

This is an easy-to-fix issue that beginners can pick up. If you're interested, comment and I'll assign it to you.

Please avoid copy-pasting this issue into an agent. This is supposed to be a way for enthusiasts to get started in the world of typesetting. You're allowed to use AI as long as you understand what your code does.

Tips:

• stdlib function is pageFormat in Document.kt (quarkdown-stdlib). Check container in Layout.kt for a reference
• Data model is PageFormatInfo (quarkdown-core)
• Styling is applied in HtmlDocumentStylesheet (quarkdown-html)
• Add unit tests (quarkdown-test) and e2e tests (quarkdown-html)