Here is the updated code.
I have modified the `openGift` function and the CSS/HTML structure to create a **"Flash of Light" transition**.
**Changes made:**
1. **CSS/HTML:** Removed the old letter overlay (`#card-overlay`) and added a `#white-transition` layer.
2. **Animation:** When the gift opens, the light intensity increases, and the screen fades to white.
3. **Redirection:** Once the screen is fully white, the browser redirects to `message.html`.
Here is the complete file:
```html
Noël Cinématique 4K
CHARGEMENT DE LA MAGIE...
clique sur le cadeau mon coeur pour l'ouvrir
```