Revamp DemosDynamic header and theming

[osalabs]

Summary

• Introduced a shared CRUD page header component and moved DemosDynamic list/view/edit to it with clearer primary/secondary actions and record navigation
• Added form section support, on-this-page structure, richer view rendering (prose + empty states), and visible autosave status handling for DemosDynamic
• Refreshed layout/theming tokens, brand component markup, and documentation while adjusting theme20/30 to rely on variable overrides

Testing

• dotnet test (fails: dotnet CLI is not available in the container)

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Codex Task