Bundle default theme assets in package

[oco777]

Summary

• move the default Unity runtime theme USS and panel theme settings asset into the package so they are always available from Runtime resources
• stop CreateProjectData from generating user-project copies of theme assets, keeping New Project focused on project-specific data
• clarify PanelSettingsInitializer fallback expectations for the packaged theme resource

Testing

• Not run (not requested)

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Codex Task