- redesign HTML entity converter with responsive UI

- implement dark/light theme with CSS custom properties
- add keyboard shortcuts (Alt+Enter, Alt+C, Alt+X, Alt+1, Alt+2)
- create tabbed interface for bidirectional conversion
- add real-time conversion option with debounced input handling
- implement toast notifications for user feedback
- add example buttons with pre-defined test cases
- improve accessibility with ARIA attributes and keyboard support
- display character counts for input and output fields
- create reference table of common HTML entities

Author: TobiasMue91