feat: add 4-step sponsor onboarding flow

[daRk8238]

Summary

New sponsors currently land directly on the dashboard with no context or guidance about how the pool works. This PR adds a 4-step onboarding flow at /sponsors/onboarding that educates new sponsors before they deposit.

Changes

New files

• src/pages/SponsorOnboarding.tsx — 4-step onboarding page with framer-motion transitions:
  1. Welcome — Explains the sponsor pool concept and how depositing funds learner loans
  2. Risks — Honest risk disclosure covering default risk, smart contract risk, liquidity risk, and protocol risk
  3. Pool Health — Live pool stats (total deposits, APY, available liquidity, locked liquidity) loaded from API via React Query, plus a yield preview calculator
  4. Deposit — Wallet connection prompt with link to GrantFox; marks onboarding as complete
• src/services/pool.service.ts — API service for fetching pool info (GET /api/v1/pool)

Modified files

• src/stores/app.store.ts — Added onboardingComplete state (persisted via zustand/persist) to track whether a user has completed onboarding
• src/router/index.tsx — Added /sponsors/onboarding route

Verification

• npm run build passes
• No any types used
• Smooth step transitions with AnimatePresence
• Yield preview calculates yearly/monthly returns from real-time APY
• Back navigation supported between steps

Closes #29