feat: add MicaAlt support for settings background (#17650)

[farheen-shaikh530]

Summary of the Pull Request

This PR adds support for a new setting useMicaAlt that enables the Mica Alt visual effect in Windows Terminal. This complements the existing useMica setting, providing more flexibility in applying Windows 11 visual styles.

References and Relevant Issues

Closes #17650
Internal discussion: [Enable support for Mica Alt in terminal settings appearance]

Detailed Description of the Pull Request / Additional comments

•	Added a new boolean property useMicaAlt in profiles.schema.json to allow users to opt into the Mica Alt effect.
•	Introduced _useMicaAlt variable and UseMicaAlt() getter in GlobalAppSettings.cpp/h.
•	Parsed the setting in LayerJson() to bind it to the global config model.
•	Extended the conditional logic in MainPage.cpp to apply the Mica Alt background when enabled.
•	This implementation mirrors the useMica structure for consistency and ease of maintenance.

No separate visual style was introduced at this stage — MicaAlt reuses the existing Mica logic as a starting point for future differentiation if needed.

Validation Steps Performed

•	Manually enabled "useMicaAlt": true in settings.json
•	Verified background behavior in the settings page correctly triggers SettingsPageMicaBackground
•	Confirmed fallback to standard background when useMicaAlt is false
•	Regression-checked useMica to ensure no interference

PR Checklist

•	Closes #17650
•	Tests added/passed (optional — maintainers may handle this)
•	Documentation updated
•	Schema updated in profiles.schema.json
•	Documentation PR: (If needed, can be filed here → https://github.com/MicrosoftDocs/terminal)

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