feat(surveys/react-native): Support custom fontFamily in survey appearance

[adboio]

Summary

The React Native SDK has built-in survey UI but doesn't apply the fontFamily from survey appearance settings.

Current State

• Survey appearance includes fontFamily in the data model
• Built-in survey components use hardcoded font styles
• fontFamily is not passed to TextStyle in any components

Expected Behavior

When a survey is configured with a custom fontFamily, the built-in survey UI should apply that font (if available/bundled in the app), falling back to system default if not found.

Reference Implementation

See posthog-ios SurveySheet.swift:

static func customFont(family: String) -> Font? {
    if let uiFont = UIFont(name: family, size: UIFont.systemFontSize) {
        return Font(uiFont)
    }
    return nil  // Falls back to system default
}

Implementation Notes

• Pass fontFamily to TextStyle in survey components
• Font must be bundled with the app to work
• Fall back to system default if font not found