[macOS 27] Fix onEditingChanged not working

[cyanzhong]

Closes #1735.

[Copilot]

🟢 Approval recommended

The change is small and directly addresses the regression by ensuring font-size updates flow through changeFontSize(_:), with only a minor accessibility improvement suggested.

Pull request overview

This PR updates the macOS Settings font-size control to reliably propagate Stepper value changes on macOS 27 (where onEditingChanged is no longer firing as expected), ensuring font-size changes are persisted via the existing handler pathway (closes #1735).

Changes:

• Replaces Stepper(..., onEditingChanged:) usage with a custom Binding setter that calls changeFontSize(_:).
• Simplifies the Stepper initializer by removing the explicit label: argument in favor of a trailing closure.

File summaries

File	Description
MarkEditMac/Modules/Sources/FontPicker/FontPicker.swift	Routes Stepper updates through a Binding setter to ensure handlers.fontSizeDidChange is called on macOS 27.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

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