Add Vitest unit tests for core utilities

[maxmini0214]

Summary

Add unit tests for the core utility functions that power DonFlow.

Priority targets

1. formatCurrency() / getCurrency() — locale-aware formatting
2. classifyMerchant() — merchant name → category mapping
3. detectCsvFormat() — auto-detection of bank CSV formats
4. calculateDrift() — plan vs actual variance calculation

Stack

• Vitest (already compatible with Vite)
• Testing Library for component tests (optional, separate issue)

Why

Core financial calculations must be correct. Tests prevent regressions on drift detection and currency formatting.