Feat: Implement Neo.form.field.CountryFlagPicker

[tobiu]

Implement a specialized form field for selecting countries with visual flag indicators, following the pattern of Neo.form.field.Color.

Requirements:

1. Base Class: Extend Neo.form.field.Country (or ComboBox).
2. Input Display: Show the selected country's flag as a circular icon inside the input field (similar to the color indicator).
3. List Items: Render the flag icon next to the country name in the dropdown list.
4. Utility: Leverage the new src/util/CountryFlags.mjs for mapping names/codes to icon paths.
5. Icons: Use the 'circle-flags' library (SVG) for the visual assets.

Acceptance Criteria:

• Users can see the flag of the selected country in the input.
• Users see flags in the picker dropdown.
• Fallback to a default icon (e.g., globe) if no flag is found.