fix(appinsights): set browser app id for correlation

[BenjaminMichaelis]

Summary\n- expose the Application Insights app id to the browser\n- configure the browser SDK with the app id for client/server correlation\n- keep correlation headers limited to the web and Try origins\n\n## Testing\n- dotnet build EssentialCSharp.Web.slnx -c Release /p:AccessToNugetFeed=false

[Copilot]

Pull request overview

This PR updates the web app’s Application Insights browser instrumentation to support improved client/server correlation by exposing the AI App ID to the browser and configuring the JS SDK to use it, while keeping correlation headers restricted to the site and Try origins.

Changes:

• Expose APPLICATIONINSIGHTS_APP_ID to the browser via _Layout.cshtml.
• Configure the Application Insights browser SDK with appId and clarify the intent of the correlation header domain allow-list.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
EssentialCSharp.Web/wwwroot/js/appinsights-manager.js	Reads the exposed App ID and passes it into the browser SDK config to support correlation; documents the restricted correlation header domain behavior.
EssentialCSharp.Web/Views/Shared/_Layout.cshtml	Adds a serialized window.APPLICATIONINSIGHTS_APP_ID value so the browser telemetry manager can configure correlation.

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