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>

backend/src/features/vehicles/domain/vehicles.service.ts

@@ -482,6 +482,10 @@ export class VehiclesService {
   private calculateMonthsOwned(purchaseDate: string): number {
     const purchase = new Date(purchaseDate);
     const now = new Date();
+    // Guard against future dates - treat as 0 months owned
+    if (purchase > now) {
+      return 0;
+    }
     const yearDiff = now.getFullYear() - purchase.getFullYear();
     const monthDiff = now.getMonth() - purchase.getMonth();
     return yearDiff * 12 + monthDiff;