chore: centralize project settings in Directory.Build.props

[United600]

Moves LangVersion and Nullable settings to Directory.Build.props to simplify configuration management across the solution. It also removes redundant LangVersion and Nullable properties from Screenbox.Core.csproj and Screenbox.csproj.

[Copilot]

Pull Request Overview

This PR centralizes project configuration by moving common compiler settings to a new Directory.Build.props file, eliminating duplication across individual project files.

• Introduces a centralized Directory.Build.props file with shared compiler settings
• Removes redundant LangVersion and Nullable properties from project files
• Cleans up unnecessary configuration blocks and file references

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
Directory.Build.props	Creates centralized configuration with LangVersion, Nullable, and AnalysisLevel settings
Screenbox/Screenbox.csproj	Removes redundant LangVersion and Nullable properties from all configuration blocks
Screenbox.Core/Screenbox.Core.csproj	Removes redundant LangVersion property
Screenbox.sln	Adds Directory.Build.props to solution items for visibility

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