feat(send): dynamically estimate and disclose network fee based on network stats (#458)

[xtep103]

Overview

Replaces the hardcoded 0.00001 XLM network fee with dynamic fee estimation based on real-time Stellar network conditions (via Horizon /fee_stats) and transaction complexity (operation count).

Related Issue

Closes #458

Changes

Send Feature & Fee Estimation

• [ADD] ignition-pay-frontend/features/send/services/fee.ts
  • Added fetchNetworkFeeStats to query /fee_stats from Horizon with graceful fallback to standard 100 stroops (0.00001 XLM) on network/server error or timeout.
  • Added estimateTransactionFee to compute dynamic fees by combining network fee stats with transaction complexity (operation count).
• [MODIFY] ignition-pay-frontend/features/send/models/index.ts
  • Added calculateTransactionFee, BASE_FEE_STROOPS, STROOPS_PER_XLM, DEFAULT_NETWORK_FEE_XLM, and NetworkFeeEstimate interface.
  • Updated spendableBalance and validateAmount to accept dynamic custom fee parameters for native XLM transactions.
• [MODIFY] ignition-pay-frontend/features/send/services/index.ts
  • Re-exported fee estimation methods and types.
• [MODIFY] ignition-pay-frontend/features/send/widgets/SendPage.tsx
  • Integrated dynamic fee estimation in the review step and confirmation step, replacing the hardcoded 0.00001 XLM.
  • Added estimation loading state with spinner in the review summary.
  • Updated max spendable balance calculation to account for dynamic network fee.
• [MODIFY] ignition-pay-frontend/components/transaction-row.tsx
  • Cleaned up unused hardcoded fee and resolved optimistic transaction ID property handling.
• [ADD] ignition-pay-frontend/tests/send-fee.test.ts
  • Added unit tests for calculateTransactionFee, fetchNetworkFeeStats, and estimateTransactionFee.
• [MODIFY] ignition-pay-frontend/tests/send-models.test.ts
  • Added test cases for dynamic fee handling in spendableBalance and validateAmount.
• [ADD] ignition-pay-frontend/tests/send-page-fee.test.tsx
  • Added component test verifying dynamic network fee retrieval and display in SendPage review step.

Verification Results

✓ tests/strkey.test.ts (12 tests)
✓ tests/send-models.test.ts (12 tests)
✓ tests/memo.test.ts (9 tests)
✓ tests/send-fee.test.ts (10 tests)
✓ tests/send-trustline.test.ts (6 tests)
✓ tests/asset-card.test.tsx (6 tests)
✓ tests/send-page-fee.test.tsx (2 tests)
✓ tests/api-keys-section.test.tsx (3 tests)
✓ tests/send-page-paste.test.tsx (7 tests)
✓ tests/dashboard-models.test.ts (8 tests)
✓ tests/accessibility.test.tsx (1 test)

Test Files  11 passed (11)
Tests       76 passed (76)

Acceptance Criteria	Status
Network fee is dynamically estimated from Horizon /fee_stats	✅ Handled with fallback to 100 stroops (0.00001 XLM)
Transaction complexity (operation count) scales fee calculation	✅ Computed via calculateTransactionFee
Review step shows dynamic fee instead of hardcoded 0.00001 XLM	✅ Integrated in SendPage review and confirmed screens
Spendable balance respects dynamic network fee allowance	✅ Updated in spendableBalance and validateAmount
Comprehensive unit and integration tests	✅ 37 send/fee tests and 76 total frontend tests passing

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