feat: migrate to shadcn toast (sonner)

- frontend/src/app.rs: Replaced custom ToastContainer with SonnerProvider
- frontend/src/store.rs: Updated show_toast to use leptos_shadcn_toast::toast API
- frontend/src/components/toast.rs: Deleted custom toast component
- frontend/src/components/torrent/add_torrent.rs: Updated toast usage
- frontend/src/components/torrent/table.rs: Updated toast usage

frontend/src/components/mod.rs

@@ -1,5 +1,4 @@
 pub mod context_menu;
 pub mod layout;
-pub mod toast;
 pub mod torrent;
 pub mod auth;