Add a Row Modifiers page to ListPlayground

[Ceylo]

This PR adds a "Row Modifiers" page to ListPlayground: rows that carry listRowBackground or listRowSeparator twice, plus a single-modifier row of each for comparison. iOS applies the innermost modifier; Android applies the outermost until skiptools/skip-ui#549.

iOS 26.5	Android, before	Android, after

Parent PR:

• Let the innermost listRow* modifier win, as SwiftUI does skip-ui#549

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Skip Pull Request Checklist:

• REQUIRED: I have signed the Contributor Agreement
• REQUIRED: I have tested my change locally with swift test. This package has no test targets. Instead, the app was built through Gradle in Lite and Fuse.
• OPTIONAL: I have tested my change on an iOS simulator or device
• OPTIONAL: I have tested my change on an Android emulator or device

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• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes.

Written with Claude Code, together with the skip-ui fix. Every claim above comes from a run: the page on an API 37 emulator (Lite and Fuse) before and after the skip-ui change, and on an iPhone 17 simulator (iOS 26.5).