fix: update AAD display name to Microsoft Entra ID in auth UI

[keith-oak]

Summary

• Updated the auth emulator UI to display "Microsoft Entra ID" instead of "AAD"
• Maintained backward compatibility by keeping 'aad' as the underlying provider value
• Added clarification in help text that "aad = Microsoft Entra ID"

Changes

• Updated datalist options in src/public/auth.html to include display text for all providers
• Changed <option value="aad"></option> to <option value="aad">Microsoft Entra ID</option>
• Updated help text to clarify the mapping

Test plan

• [x] Built the project successfully
• [x] All unit tests pass
• [x] The UI now shows "Microsoft Entra ID" in the dropdown while still using 'aad' as the value
• [x] No breaking changes - existing configurations and URLs continue to work

Fixes #850

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