Improve CountrySearch UX & keyboard accessibility

[adiancodes]

• Replaced window.onload with DOMContentLoaded for better event timing and performance

• Added Escape key support to close the suggestion dropdown

if (event.key === 'Escape') {
    this.clearSearchDropdown();
    this.searchInput.blur();
}

This commit updates the autocomplete search and region filtering logic with better performance and accessibility support.