Use LiveToast instead of built-in flash helpers

So that they can stack and get dismissed automatically

lib/music_library_web/controllers/last_fm_controller.ex

@@ -7,12 +7,12 @@ defmodule MusicLibraryWeb.LastFmController do
     with {:ok, session} <- LastFm.get_session(token),
          {:ok, _secret} <- Secrets.store("last_fm_session_key", session.key) do
       conn
-      |> put_flash(:info, gettext("Successfully connected your Last.fm account"))
+      |> put_toast(:info, gettext("Successfully connected your Last.fm account"))
       |> redirect(to: ~p"/")
     else
       {:error, reason} ->
         conn
-        |> put_flash(:error, "Failed to connect your Last.fm account: #{reason}")
+        |> put_toast(:error, "Failed to connect your Last.fm account: #{reason}")
         |> redirect(to: ~p"/")
     end
   end

lib/music_library_web/controllers/session_controller.ex

@@ -22,7 +22,7 @@ defmodule MusicLibraryWeb.SessionController do
       |> redirect(to: ~p"/")
     else
       conn
-      |> put_flash(:error, gettext("Invalid password"))
+      |> put_toast(:error, gettext("Invalid password"))
       |> redirect(to: ~p"/login")
     end
   end