feat: add username and display it in header

pento/priv/repo/migrations/20240818133031_add_username.exs

@@ -0,0 +1,11 @@
+defmodule Pento.Repo.Migrations.AddUsername do
+  use Ecto.Migration
+
+  def change do
+    alter table("users") do
+      add :username, :string, null: false
+    end
+
+    create unique_index(:users, [:username])
+  end
+end