Wire real Horizon USDC balance into WalletContext

[go165]

Fixes #126

This replaces the mocked wallet balance with a Horizon-backed USDC lookup for the connected Freighter account. It adds loading/error/trustline states in the wallet dropdown, keeps the behavior network-aware, and documents the balance flow in the README.

Verification:

• npm test -- --run src/utils/tests/horizon.test.ts src/context/tests/WalletContext.test.tsx src/pages/tests/Home.test.tsx
• npx tsc --noEmit --jsx react-jsx --target ES2020 --module ESNext --moduleResolution bundler --lib ES2020,DOM,DOM.Iterable --strict --skipLibCheck src/utils/horizon.ts src/context/WalletContext.tsx src/components/Wallet/WalletDropdown.tsx src/components/Field.tsx

Repo-wide build still fails on pre-existing unrelated TypeScript issues in App/Layout/Vault/Vaults test files.

Payout address: 0x1f0130669ca6fd02e025a984cc038f139df19a2f