feat: Integrate Cross-Asset Payment UI with pathfinding and Soroban (#75)

[Uchechukwu-Ekezie]

Resolves #76

Changes:

• Created backend proxy endpoint GET /api/payments/paths to query Stellar Horizon strictSendPaths with dynamic source/dest assets and amounts.
• Created pathfindingService.ts in the frontend to correctly fetch viable cross-asset routes using the new backend proxy.
• Rewrote CrossAssetPayment.tsx to completely drop the mocked Anchor integration and use the real paths response.
• Wired debounced inputs for fetching quotes automatically.
• Integrated @creit.tech/stellar-wallets-kit via WalletProvider to prompt actual wallet signing for the Soroban cross_asset_payment smart contract.
• Added dynamic status updates via mocked polling until Socket indexers are verified.

Tested:

• Backend proxy types pass compilation on main.
• Frontend TS compilation succeeds.