feat(lib): format stroop values with locale grouping

[YfengJ]

Closes #74.

Summary

• update formatStroops to use Intl.NumberFormat with deterministic en-US grouping
• keep 0 XLM, preserve the public formatStroops(stroops: number): string signature, and show non-zero sub-cent values as grouped raw stroops instead of hiding them behind 0.00 XLM
• format XLM values with at least 2 and up to 7 fraction digits so large and fractional balances stay readable
• route formatRequests through the same grouped integer formatter
• update README formatting notes and affected tests
• clean up two existing test/lint blockers so the repo-level validation command can run green

Validation

• npm run lint
• npm run typecheck
• npm test -- --runInBand (43 suites, 334 passed, 2 skipped; existing React act warning is still printed by AgentsPage tests)
• npm run build
• npm test -- src/lib/__tests__/format.test.ts --runInBand --coverage --collectCoverageFrom=src/lib/format.ts --coverageThreshold='{"global":{"statements":0,"branches":0,"functions":0,"lines":0},"src/lib/format.ts":{"statements":95,"branches":90,"functions":95,"lines":95}}'

format.ts coverage from the targeted coverage run:

• Statements: 100%
• Branches: 93.18%
• Functions: 100%
• Lines: 100%

[mikewheeleer]

clean diff — thanks for this!