Implement locale-aware formatting for dates and amounts

[sitarua]

Closes #131

Summary

• centralize browser-locale formatting helpers for money, plain numbers, percentages, and dates
• replace hard-coded dollar/date/percent formatting in dashboard, bills, budgets, reminders, and chart tooltips
• document the shared formatting helpers and add focused formatter tests

Tests

• npm test -- --runTestsByPath src/__tests__/currency.test.ts
• npm run build