Refresh site branding: logo, glow, favicons, and dark defaults

[aivascu]

Summary

• Flip logo wordmark colors so Fixture is primary green, and add light/dark favicons that follow the OS theme
• Restyle the Open Graph image for dark mode with a top green glow
• Add shared page top glow, neon landing code-block glow, GitHub star count in the header, and AutoFixture in tab titles
• Prefer system color mode with dark as the fallback when no OS preference is available

Test plan

• Landing page: logo colors, neon code-block glow, top edge glow, and radial hero glow look correct in light and dark
• Docs/API pages: shared top glow appears; browser tab titles include AutoFixture
• Toggle OS light/dark: favicon switches; with no saved theme preference, color mode follows system and falls back to dark
• Header GitHub button shows compact star count (for example 3.5K) and links to the main repo
• Open Graph preview uses the dark og-image (for example via a social debugger or /og-image.png)