Enable VPS demo mode with local authentication

[mudabs]

Closes #1

Summary

• Replace AD credential validation and the global AD group authorization policy with local cookie authentication.
• Configure the demo username/password through appsettings or environment variables.
• Remove the unused Negotiate package.
• Store data-protection keys in a configurable, VPS-safe application directory.
• Stop forcing trusted Windows connections in the sample SQL connection string.
• Keep Active Directory synchronization services available as optional administrative functionality.

Verification

• dotnet restore
• dotnet build --no-restore (passes with 0 errors; existing warnings remain)

Demo defaults:

• Username: demo
• Password: ChangeMe123!

Replace these values through DemoAuthentication__Username and DemoAuthentication__Password before public deployment.

[coderabbitai]

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