style: update typography and visual design across the application

- Change font-weight from semibold to medium for headings
- Increase h1 font-size from 3xl to 4xl for better hierarchy
- Update button and link text to sentence case for consistency
- Change code syntax highlighting theme from github-dark to vesper
- Replace Charter font with IBM Plex Serif from Google Fonts
- Update Vercel logo to simplified version without text
- Add cyan theme parameter to star history chart
- Refine stars count typography with text-box trim
- Adjust testimonial card text styling with text-pretty
- Update GitHub icon SVG path for better rendering

feat: add keyboard shortcut for theme toggle

- Add react-hotkeys-hook dependency
- Implement 'd' key shortcut to switch between light and dark themes

refactor: improve theme toggle implementation

- Extract theme switching logic into switchTheme callback
- Remove commented SVG code for cleaner component