feat: implement wallet transaction history with Stellar Horizon (#63)

[tarai-dl]

Summary

Implements on-chain transaction history fetching and display using the Stellar Horizon API.

Changes

• **** — New Horizon API client with:

  • Cursor-based pagination for navigating transaction pages
  • In-memory cache with 30-second TTL to reduce API calls
  • Operation enrichment (fetches operations per transaction)
  • Typed interfaces for transactions, operations, and pages
  • Configurable via NEXT_PUBLIC_HORIZON_URL and NEXT_PUBLIC_CONTRACT_ID env vars

• **** — New transaction history page with:

  • Loading skeletons while fetching
  • Operation badges (Payment, Contract Invoke, etc.) with color coding
  • Truncated hashes and addresses for readability
  • Previous/Next pagination controls
  • Refresh button with loading animation
  • Empty state handling
  • Error state display

• **** — Added Transactions navigation link

Acceptance Criteria

• Query Horizon API for transactions involving the contract
• Parse operation types (payments, contract invocations)
• Display in a formatted transaction list
• Add pagination for large histories
• Cache results to reduce API calls

Closes #63