fix(Toaster): smoother visibility transition for stacked toasts

[sifferhans]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

The content of stacked toasts are hidden when not in the front of the stack, but in my opinion the transition between the visible and invisible states are a bit abrupt. See how the content of the stacked toasts disappear before they are collapsed:

CleanShot.2025-06-17.at.12.33.30.mp4

I switched to using opacity-0 instead of invisible, and I added a slight transition.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxt/ui@4367

commit: 904163c

[benjamincanac]

Thanks, looks better indeed 😊