Add build/commit identity to standalone page and diagnostics

[Stephen Griffin (stephenegriffin)]

• Introduce BuildInfo module with buildNumber, commit, and builtAt, populated from MHA_BUILD_NUMBER and SCM_COMMIT_ID or from local git HEAD when unset. Fail fast on missing/malformed pipeline input.
• Emit Pages/build-info.json from the same metadata so support can verify a deployed build without loading the app.
• Add a small About dialog to the standalone page showing build, commit (linked to GitHub), and built-at; move the feedback link into it.
• Diagnostics now report Build, Commit/Base commit, and Built, replacing the old mhaVersion hash and Last Update.
• Remove the custom commit-hash asset directory; slot swap and existing Web.config no-cache handle deployment identity.
• Drop buildTime.ts, mhaVersion.ts, and their DefinePlugin globals.

[Copilot]

Pull request overview

This PR adds first-class build identity metadata (build number, commit SHA, built-at timestamp) to the standalone experience and diagnostics, and exposes the same metadata as a deploy-time JSON artifact for supportability.

Changes:

• Inject a mhaBuildInfo global at build time (from pipeline env or local Git HEAD) and emit Pages/build-info.json.
• Add a standalone “About” dialog that surfaces build/commit/built-at (with commit linking to GitHub).
• Replace prior mhaVersion / Last Update diagnostics fields with build/commit/built-at fields and remove old version/build-time modules.

Reviewed changes

Copilot reviewed 14 out of 15 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
webpack.config.js	Computes build metadata, injects mhaBuildInfo, emits build-info.json, and removes hashed asset paths.
src/Scripts/ui/StandaloneAbout.ts	New About dialog wiring for standalone page using injected build metadata.
src/Scripts/ui/StandaloneAbout.test.ts	Adds unit test coverage for About dialog behavior.
src/Scripts/ui/mha.ts	Loads fluent dialog component and initializes the About dialog on DOM ready.
src/Scripts/mhaVersion.ts	Removes old DefinePlugin-driven version hash module.
src/Scripts/Diag.ts	Switches diagnostics to report build/commit/built-at via BuildInfo.
src/Scripts/buildTime.ts	Removes old DefinePlugin-driven build time module.
src/Scripts/BuildInfo.ts	New module providing typed accessors for build metadata and commit URL.
src/Scripts/BuildInfo.test.ts	Adds unit test coverage for the BuildInfo helpers.
src/Pages/mha.html	Adds About dialog markup and replaces the old feedback link placement.
src/Content/classicDesktopFrame.css	Styles the About link/button and dialog layout.
README.md	Documents new build identity behavior and the /Pages/build-info.json endpoint.
jest.config.ts	Adds Jest globals for mhaBuildInfo to run tests outside webpack.
index.d.ts	Declares the mhaBuildInfo global for TypeScript.
eslint.config.js	Updates ESLint globals to reflect the new webpack-defined constant.

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