- completely redesign UI with responsive layout and dark mode support

- add world clocks feature with persistent favorites via localStorage
- implement time difference calculation between zones
- add timezone search functionality with filtering
- improve time input with separate hour/minute/second fields
- add timezone swap button for quick conversion reversal
- enhance date handling to show date differences when converting across days
- replace static timezone buttons with dynamic timezone selector populated from moment.js data

Author: TobiasMue91