fix: stabilize repeated lifecycle effects in landing experience

[amberly-d]

Description

Extracts inline timers, scroll listeners, and fetch/polling logic from landing page components into dedicated reusable hooks with proper cleanup semantics and unmount guards.

Closes #19

Key Changes

• useVerificationSimulation hook: Extracts hero section timer logic (step progression, verified state) with isMounted guard and interval cleanup
• useLeaderboardData hook: Extracts leaderboard fetch + polling logic with AbortController cancellation and isMounted guard
• useScrolled hook: Extracts scroll position tracking with passive listener and cleanup
• Refactored components: HeroSection, LeaderboardSection, Navbar now use the new hooks instead of inline effects
• 13 unit tests: Covering timer progression, cleanup on unmount, fetch/polling behavior, scroll threshold

Testing

• npm run test — all 13 new lifecycle hook tests pass, all existing tests unaffected
• npm run build — only pre-existing import.meta.vitest type error in errorHandler.ts

Type of Change

• Bug fix (non-breaking change which fixes an issue)