Fix: Default option of Font Appearance in Typography Panel

[Debarghya-Banerjee]

What?

Closes #70938

Fixes the issue where resetting typography and selecting Default from the Font Appearance dropdown in the Site Editor was not possible.

Why?

The previous solution used a useEffect that ran on changes to nearestFontStyle and nearestFontWeight only. It updated or reset font appearance regardless of whether the font family changed or if the user had explicitly set font style or weight. This caused:

• Unwanted resets or overwrites of user choices.
• Incorrect behaviour when selecting Default, as the default value (undefined) was not handled properly.
• Font appearance being stuck or falling back unexpectedly.

How?

• Introduced a useRef to track the previous font family, allowing the effect to only run when the font family actually changes.
• Added logic to check if the user has explicitly set font style or weight, preventing unintended overrides.
• When the font family changes and there is no explicit font appearance set, the nearest valid font style and weight are applied.
• If no valid styles or weights are available for the new font family, the font appearance is reset cleanly.

[github-actions]

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Co-authored-by: Debarghya-Banerjee <debarghyabanerjee@git.wordpress.org>

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