Enhance README logo display for light and dark mode support

feat: Add dark mode logo support ## Changes - Added white logo variant for dark mode (`assets/logo-darkmode.svg`) - Updated README.md to use `<picture>` element for theme-aware logo display - Maintains original logo for light mode and fallback ## Technical Details - Uses `prefers-color-scheme` media query for automatic theme detection - Preserves original logo dimensions and alignment - Ensures backwards compatibility with browsers not supporting `<picture>` ## Testing - Verified logo visibility in both light/dark modes - Tested fallback behavior - Confirmed responsive sizing maintained