feat(surveys): Support custom fontFamily in survey appearance

Summary

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

Current State

• PostHogDisplaySurveyAppearance includes fontFamily field
• Built-in survey widgets use hardcoded TextStyle without fontFamily
• The data is parsed but not used

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 widgets (e.g., question_header.dart, survey_button.dart, etc.)
• Font must be declared in pubspec.yaml and bundled with the app
• Fall back to system default if font not found

Start by reviewing the survey widgets named in the issue, including question_header.dart and survey_button.dart, and trace how PostHogDisplaySurveyAppearance.fontFamily reaches them. Apply the configured family through each relevant TextStyle, preserving the system default when the font is unavailable. Confirm the behavior with a bundled font and with an unavailable font.