frontend: ProofHistory.tsx component exists but is never rendered in App.tsx

[IyanuOluwaJesuloba]

ProofHistory.tsx is defined but never imported or used in App.tsx. A proof history tab would let users see their previous proof submissions and tier upgrades, which is useful UX for the hackathon demo.

Acceptance criteria:

• Add a "History" tab in App.tsx that renders

• ProofHistory reads past tier_set contract events from the registry via Horizon's event endpoint or local storage as a fallback

• Tab only appears when wallet is connected