fix: make watch demo button scroll to testimonials

[gTanusri]

Summary

• Add click behavior to the hero "Watch Demo" button so it smoothly scrolls to the testimonials section
• Update the button to match the primary CTA styling with pointer, hover scale, and focus states
• Keep the active nav state in sync when the demo button is used

Tests

• npm run build fails due to existing parse error in src/services/public/ApiService.js, unrelated to this change
• .\node_modules\.bin\eslint src\pages\public\Landing.jsx passes with existing warnings for navLinks dependency and unused stats

Closes #64