fix: replace mock trustline check with Horizon/backend queries

[Marvy54]

Overview

This PR replaces the mocked checkTrustline implementation in the send flow with a real trustline verification service. It queries Horizon for the account’s trustlines and falls back to the configured backend API when needed, ensuring users get accurate trustline status before sending assets.

Related Issue

Closes: Trustline check uses mock service

Changes

🔍 Real Trustline Verification

• [MODIFY] features/send/services/trustline_service.dart

  • Remove the mock checkTrustline logic; add a network-based implementation that queries Horizon for trustlines.
  • Add graceful fallback to the backend endpoint when Horizon is unreachable.
  • Include proper error mapping for missing accounts, invalid assets, and network failures.

• [MODIFY] .env.example

  • Add HORIZON_URL and TRUSTLINE_BACKEND_URL environment variables for configuring the trustline check endpoints.

• [MODIFY] examples/dart-wallet/basic-wallet/main.dart

  • Update the example wallet to initialize the real trustline service and display actual trustline results.

• [MODIFY] examples/dart-wallet/basic-wallet/pubspec.yaml

  • Add http dependency required for the network-backed trustline service.

• [MODIFY] examples/ts-backend/stellar-api-server/src/index.ts

  • Add a /trustline endpoint that verifies trustlines via Horizon and returns a normalized JSON response for the Flutter client.

Verification Results

flutter test --coverage
✅ 24/24 tests passed

Manual verification:
✅ checkTrustline now queries live Horizon testnet
✅ Backend fallback returns correct trustline status when Horizon is down
✅ Invalid asset code returns a clear error
✅ Example wallet shows real trustline status

Acceptance Criteria	Status
checkTrustline queries Horizon instead of returning simulated results	✅
Backend fallback works when Horizon is unavailable	✅
Trustline errors are propagated clearly to the user	✅
Configuration is documented via .env.example	✅

Closes #455

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