feat: add fund flow export endpoint

[Vinayak1337]

Fixes #79

What changed

• Added an account-service method for exporting fund flow.
• Added a signed GET /openApi/swap/v2/user/income/export endpoint with optional filters.
• Set export responses to request arraybuffer because BingX returns an Excel file.
• Kept binary export responses out of the shared JSON parser so expected Excel responses pass through without parse-error logging.
• Added focused tests for request shape and optional parameter handling.
• Added focused coverage for JSON parsing and binary response passthrough.
• Marked the README feature checkbox as supported.
• Removed one unused import so lint passes.

Tests

• npm run lint
• npm test -- account.service.spec.ts bingx-request.spec.ts
• npm run build
• Manual local harness: TS_NODE_PROJECT=tsconfig.json npx ts-node -T -r tsconfig-paths/register ../bounty-hunt-tracker/manual-harnesses/pr96-export-fund-flow.ts

Notes

Full npm test was attempted, but existing playground specs require API_ACCOUNT_KEY / API_ACCOUNT_SECRET, and an existing socket spec timed out in the full run.