Show whether we're using local eval or not

[haacked]

Update the sample to show whether local eval is being used or not.

[greptile-apps]

PR Summary

This PR enhances the visibility of PostHog's feature flag evaluation mode in the sample web application by adding clear indicators for local vs. remote evaluation states.

• Added new PersonalApiKeyIsSet property in Index.cshtml.cs to track local evaluation capability
• Updated alert messages in Index.cshtml to explicitly show whether local or remote evaluation is being used
• Renamed ApiKeyIsSet to ProjectApiKeyIsSet for better clarity on API key types
• Improved error handling by separating project and personal API key validation checks

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