[WIP] Fix feedback on sticky header implementation for PR overview

[Copilot]

• Add IntersectionObserver mock to the test environment
• Update test to verify sticky header visibility changes when IntersectionObserver triggers
• Test the complete sticky behavior flow (initial state → scrolled state → back to top)
• Use proper TypeScript types (SinonStub instead of any)
• Fix typos in type definition files
• Final security check completed - no vulnerabilities found

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.