Add Vereinfacht app URL setting and contact view URL

- Setting attribute vereinfacht_app_url, migration, .env.example
- Config: vereinfacht_app_url() from env/setting or derived from API URL
- Contact view URL uses app URL with /en/admin/finances/contacts/{id}
- Global settings: App URL field, read-only when VEREINFACHT_APP_URL set
- Tests: update contact view URL expectations

priv/repo/migrations/20260218190000_add_vereinfacht_app_url.exs

@@ -0,0 +1,15 @@
+defmodule Mv.Repo.Migrations.AddVereinfachtAppUrl do
+  use Ecto.Migration
+
+  def up do
+    alter table(:settings) do
+      add :vereinfacht_app_url, :text
+    end
+  end
+
+  def down do
+    alter table(:settings) do
+      remove :vereinfacht_app_url
+    end
+  end
+end