feat: wallet transaction history with Stellar Horizon API

[victorjzq]

/claim #63

Summary

Implements on-chain transaction history for connected wallets using the Stellar Horizon REST API, as specified in issue #63.

New files

src/lib/horizon.ts — Horizon API client:

• fetchAccountTransactions(accountId, network, cursor) — paginated operations for any Stellar account
• fetchContractTransactions(contractId, network, cursor) — operations for Soroban contract addresses
• Full TypeScript types for Horizon operations and parsed results
• Cursor-based pagination following HAL _links from Horizon responses
• 30-second in-memory cache keyed by network:account:cursor to reduce redundant requests
• Operation type normalisation (payment, invoke_host_function, path_payment, etc.)
• Stellar Expert explorer URL generation for both Testnet and Mainnet

src/app/transactions/page.tsx — /transactions page:

• Table listing type, date, amount/asset, and linked transaction hash
• Testnet / Mainnet network switcher dropdown
• Previous / Next pagination with page counter
• Loading spinner, error banner with retry button, empty-state fallback
• Graceful handling of accounts not yet on-chain (404 → empty list)
• Wallet-not-connected guard with prompt

Modified files

src/components/Navbar.tsx — adds a "Transactions" link to the nav bar.

Acceptance Criteria

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

Test plan

• Connect Freighter wallet on Testnet → transactions table populates
• Switch to Mainnet → table reloads for mainnet
• Click Next / Previous → page advances and retreats correctly
• Disconnect wallet → empty-state message shown
• Account with no history → "No transactions found" shown