Add comprehensive SendTokensScreen tests (Closes #82)

[Codex723]

Title: Add comprehensive test coverage for SendTokensScreen

Closes #82

Summary

This PR adds full test coverage for src/screens/SendTokensScreen.tsx, which was previously untested despite handling the entire payment flow. It covers input validation, destination/amount/asset handling, the Lobstr delegation path vs. the in-app signing path, error handling, and success confirmation display.

Changes

• New test file: src/__tests__/SendTokensScreen.test.tsx with 19 test cases (acceptance criteria required 10+).

Coverage highlights

• Rendering — in-app vs. Lobstr variants (subtitle + "Send" / "Send via Lobstr" button label).
• Input validation — invalid destination address, invalid amount, destination validated before amount, and a no-op when no wallet is connected.
• Lobstr delegation — opens the SEP-7 pay URI with trimmed args, shows the "Payment opened in Lobstr" confirmation alert, clears the inputs, and refreshes balances after the 3s delay (fake timers); error path when LobstrNotInstalledError is thrown.
• In-app signing — getInAppSecret + signAndSubmitPayment called with the correct params, success alert with the truncated transaction hash, balance refresh, and input clearing; missing-secret-key error path.
• Error handling — network failures surface the thrown message; op_no_trust maps to the friendly trustline message.
• Asset selection/config — ECO and USDC assets wired to their configured issuers, plus both "not configured" error branches.
• UI state — spinner + disabled send button while a submission is in flight, and Cancel → goBack.

Testing

• Targeted suite: 19/19 tests passed
• Full suite (npm test): 43 suites / 380 tests passed
• Typecheck (tsc --noEmit): clean (0 errors)
• ESLint: 0 errors (only the same no-floating-promises warnings already present in existing tests)

Notes

• Mocking follows the repo's existing conventions (jest.mock for useWalletStore, useStellarWallet, getInAppSecret, signAndSubmitPayment, isValidAmount, isValidPublicKey, openLobstrForPayment/LobstrNotInstalledError, @react-navigation/native; Alert spy; react-test-renderer + act).
• react-native-config is mocked with inline values (matching useStellarWallet.test.tsx) so the ECO/USDC asset paths can run.
• No production/business logic was changed — this is a test-only PR.

[cybermax4200]

your branch naming does'nt follow the repo convention