Migrer le toaster 🍞

[abulte]

@datagouv/components-next v1.0.0 migrated its internal toast system from vue3-toastify to vue-sonner, re-exporting Toaster and toast from that package (opendatateam/udata-front-kit#1074).

Our app still uses vue3-toastify (src/utils/error.ts, DatasetAddToTopicModal.vue). Until we migrate, components-next toasts (e.g. the "Copied!" feedback in CopyButton) are silently dropped because is never mounted.

Migration steps:

1. Replace vue3-toastify imports in error.ts and DatasetAddToTopicModal.vue with toast from @datagouv/components-next
2. Remove import 'vue3-toastify/dist/index.css' from main.ts
3. Mount (from @datagouv/components-next) in the app layout
4. Update --toastify-* CSS variables in main.css to sonner equivalents
5. Remove vue3-toastify dependency