redesign the repository banner with a cleaner GitHub-style visual identity (#211)

- replaced the original flat background with a modern gradient composition
- updated title typography using Hubot Sans (GitHub official font)
- used Segoe UI for supporting text to improve readability and hierarchy
- improved spacing, composition, and overall visual balance
- integrated the Securitocat mascot for a more playful GitHub-inspired aesthetic

mascot source:
https://octodex.github.com/securitocat/