🏗️ Vanguard: Button Drop Shadow Parity Integration

[tedd]

💡 Objective: The underlying Button component in the core library supports a classic DOS-style drop shadow (via ShadowStyle and ShadowBackground), but this functionality was not exposed in the declarative Blazor wrapper (TuiButton) and was not showcased symmetrically across demonstration targets.
🎯 Execution: Exposed ShadowStyle and ShadowBackground on TuiButton. Updated all four deployment targets (demo.xaml, Program.cs, Home.razor, and Programmatic.razor) to configure the "Show Dialog" button with a Solid shadow and ConsoleColor.DarkGray background.
📊 Functional Impact: Ensures unified cross-platform capability by making advanced button styling parameters declaratively accessible in Razor configurations, fully syncing the demonstration outputs.
🔬 Verification Protocol: Build solutions; run programmatic and declarative outputs locally; verify the modal trigger button renders an identical dark gray solid shadow underneath it on all 4 interfaces.

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PR created automatically by Jules for task 14425927212158530825 started by @tedd

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