- modernize age calculator UI with responsive design

- add dark mode with theme persistence
- implement loading animation for better UX
- enhance results display with detailed age breakdown
- add zodiac sign information and birthday countdown
- add `copyResults()` function for sharing
- replace fixed width design with fluid container
- improve accessibility with proper HTML5 structure
- use CSS variables for consistent theming
- fix age calculation logic with proper month/day handling

Author: TobiasMue91