feat: add ownership cost fields to vehicle form (refs #15)

- Add CostInterval type and TCOResponse interface
- Add TCO fields to Vehicle, CreateVehicleRequest, UpdateVehicleRequest
- Add "Ownership Costs" section to VehicleForm with:
  - Purchase price and date
  - Insurance cost and interval
  - Registration cost and interval
  - TCO display toggle
- Add getTCO API method
- Mobile-responsive grid layout with 44px touch targets

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

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

frontend/src/features/vehicles/api/vehicles.api.ts

@@ -3,7 +3,7 @@
  */
 
 import { apiClient } from '../../../core/api/client';
-import { Vehicle, CreateVehicleRequest, UpdateVehicleRequest, DecodedVehicleData } from '../types/vehicles.types';
+import { Vehicle, CreateVehicleRequest, UpdateVehicleRequest, DecodedVehicleData, TCOResponse } from '../types/vehicles.types';
 
 // All requests (including dropdowns) use authenticated apiClient
 
@@ -88,5 +88,13 @@ export const vehiclesApi = {
   decodeVin: async (vin: string): Promise<DecodedVehicleData> => {
     const response = await apiClient.post('/vehicles/decode-vin', { vin });
     return response.data;
+  },
+
+  /**
+   * Get Total Cost of Ownership data for a vehicle
+   */
+  getTCO: async (vehicleId: string): Promise<TCOResponse> => {
+    const response = await apiClient.get(`/vehicles/${vehicleId}/tco`);
+    return response.data;
   }
 };