- upgrade file size converter with responsive design and dark mode support

- add toggle between binary (1024) and decimal (1000) conversion units
- implement configurable decimal precision for conversion results
- add clipboard functionality with modern `navigator.clipboard` API
- create conversion history feature with localStorage persistence
- improve error handling and validation for input values
- optimize mobile responsiveness with flexbox layouts
- add comprehensive meta tags for SEO and social sharing
- implement CSS variables for theming and consistent spacing
- replace imperative code with IIFE pattern for better encapsulation

Author: TobiasMue91