Add Medium-Small font size option

[maxistar]

Motivation

• Provide an intermediate font size between Small and Medium so users have a finer-grained sizing option.

Description

• Add a new setting constant SETTING_MEDIUM_SMALL = "Medium Small" in SettingsService.java and use that as the label for the new size.
• Integrate the new size into the font size picker in FontSizePreference.java by adding it to the displayed list, updating selection index handling, and setting the preview text size to 18.0f.
• Apply the new size in the editor rendering by handling SETTING_MEDIUM_SMALL in EditorActivity.applyFontSize() and mapping it to mText.setTextSize(18.0f).

Testing

• No automated tests were run for this change.

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Codex Task