Refactor JavaScript to use querySelector for robustness. Update HTML
form to use POST method and correct script loading. Fix message div
syntax for better accessibility and compatibility.
Refactored web server to move route registration into a dedicated method
and extracted static CSS into its own file for better maintainability.
Updated HTML template to use external stylesheet and improved JS code
style consistency. This enhances separation of concerns and prepares
for easier future asset management.
