Theme browser: prevent previous navigation on first theme.

[afercia]

Trac ticket: https://core.trac.wordpress.org/ticket/65715

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[github-actions]

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[Copilot]

Pull request overview

Prevents unintended “wrap-around” behavior in the Theme Browser overlay when navigating to the previous theme while already on the first theme in the collection.

Changes:

• Adds an early return in the previous() navigation handler when the current theme is at index 0.

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[afercia]

Hm I'm not sure whether the 'not found' scenario is a real case. Would appreciate feedback from someone more familiar with the themes collection. Granted, <= 0 would not harm.

[github-actions]

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