feat(transactions): wallet transaction history via Horizon

[aunysillyme]

Summary

Closes #63.

Adds a new /transactions page that fetches on-chain transaction history for the SoroSave contract directly from the Stellar Horizon API.

• src/lib/horizon.ts — fetch()-based Horizon client (no extra SDK pulled in). Exposes:
  • fetchTransactionsForAccount(address, { limit, cursor, order })
  • fetchOperationsForAccount(address, { limit, cursor, order }) (includes failed ops)
  • Typed models for transactions / operations and a formatOperationType + summarizeOperation pair for rendering payments, contract invocations, path payments, trustline changes, offers, TTL extensions, etc.
  • 30s in-memory response cache keyed by URL to keep repeated paging cheap.
• src/app/transactions/page.tsx — paginated transaction list. Reads NEXT_PUBLIC_CONTRACT_ID from the env (with an inline address override input for ad-hoc browsing), shows operation type, summary, relative timestamp, success/failure, and a deep link to the Horizon tx page. ← Newer / Older → buttons walk forward and back through cursor pages.
• Navbar — adds a Transactions link.

Horizon URL defaults to https://horizon-testnet.stellar.org and is overridable via NEXT_PUBLIC_HORIZON_URL.

Acceptance criteria (from issue)

• 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

Test plan

• pnpm dev and navigate to /transactions — list renders, pagination works, deep links open Horizon
• Without NEXT_PUBLIC_CONTRACT_ID set, the page prompts for an address and the inline input loads it
• Set NEXT_PUBLIC_HORIZON_URL to mainnet Horizon and confirm it queries the right host
• Repeated page visits within 30s do not re-hit Horizon (cache verified via network panel)

Notes

This PR is AI-assisted (Claude Code). Author reviewed the diff before opening.

/claim #63