Fix: prevent double POST submission in shadcn ActionButtons

Summary

• Add type="button" to all buttons in action-buttons.tsx
• shadcn <Button> defaults to type="submit", causing double POST when rendered inside a <form> — the onClick handler fires formik.submitForm() AND the native form onSubmit triggers formik.handleSubmit

Affected

All forms using shadcn ActionButtons inside a <form> wrapper (Marque/Modèle, Fiche Véhicule, event details, etc.)