- redesign l33tspeak converter UI with modern styling system

- implement dark mode with system preference detection
- add conversion history tab with localStorage persistence
- add keyboard shortcuts (Alt+Enter, Alt+C, Alt+1-4)
- implement toast notification system for user feedback
- create preset configurations (hacker, gamer, extreme, secret)
- add help section with keyboard shortcuts and tips
- improve custom replacements UX with dedicated form fields
- optimize mobile responsiveness with flexible layouts
- add text statistics with leet level indicator

Author: TobiasMue91