fix: add TCO unit tests and fix blocking issues (refs #15)

Quality Review Fixes:
- Add comprehensive unit tests for getTCO() method (12 test cases)
- Add tests for normalizeRecurringCost() via getTCO integration
- Add future date validation guard in calculateMonthsOwned()
- Fix pre-existing unused React import in VehicleLimitDialog.test.tsx
- Fix pre-existing test parameter types in vehicles.service.test.ts

Test Coverage:
- Vehicle not found / unauthorized access
- Missing optional TCO fields handling
- Zero odometer (costPerDistance = 0)
- Monthly/semi-annual/annual cost normalization
- Division by zero guard (new purchase)
- Future purchase date handling

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/shared-minimal/components/VehicleLimitDialog.test.tsx

@@ -3,7 +3,6 @@
  * @ai-context Validates props, mobile/desktop modes, and user interactions
  */
 
-import React from 'react';
 import { render, screen, fireEvent } from '@testing-library/react';
 import '@testing-library/jest-dom';
 import { VehicleLimitDialog } from './VehicleLimitDialog';