Fix JMD display precision handling in wallet transaction mapping

[temp4allOG]

Summary

Backend-side fixes for JMD precision and display consistency from issue #282.

Included changes

• src/app/wallets/get-transactions-for-wallet.ts
  • stop hardcoding displayCurrency: "USD"; now uses transaction display currency when available
  • replace Math.floor(...) with Math.round(...) for exchange-rate base conversion to avoid systematic downward truncation
  • prefer settlementDisplayAmount / settlementDisplayFee values from upstream transaction payload when provided
• src/graphql/shared/types/object/btc-wallet.ts
  • normalize BTC wallet balance before exposing via GraphQL resolver to avoid object-like serialization issues

Notes

• This PR intentionally focuses on backend fixes in lnflash/flash.
• Mobile-side changes mentioned in the issue belong to lnflash/flash-mobile and should be handled in a paired PR.

Validation

• Dependency install completed successfully with engine override in this environment.
• Typecheck in this environment reports several unrelated pre-existing repo errors under Node 22 / local setup; no new compile errors were introduced in the touched files.

BTC fallback address: 1BL4eV82zZ64Dp4cj3s9EgJ3ae8xPx5ZuJ