VSN-2829: add vehicleAge (ancienneté) field to vehicle detail and creation form

• Added getBrandModelByBrandAndModel() to the brand-model store to fetch the full VehicleBrandModel object (including vehicleAge) by brand and model name
• Displayed "Ancienneté (année)" as a read-only required field in the vehicle detail card, positioned right after the "Type de véhicule" field, using shadcn Label + Input components for design consistency