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

test/mv/config_vereinfacht_test.exs

@@ -39,14 +39,23 @@ defmodule Mv.ConfigVereinfachtTest do
       assert Mv.Config.vereinfacht_contact_view_url("123") == nil
     end
 
-    test "returns URL when API URL is set" do
+    test "returns app contact view URL when API URL is set (derived app URL)" do
       set_vereinfacht_env("VEREINFACHT_API_URL", "https://api.example.com/api/v1")
 
       assert Mv.Config.vereinfacht_contact_view_url("42") ==
-               "https://api.example.com/api/v1/finance-contacts/42"
+               "https://app.example.com/en/admin/finances/contacts/42"
     after
       clear_vereinfacht_env()
     end
+
+    test "returns app contact view URL when VEREINFACHT_APP_URL is set" do
+      set_vereinfacht_env("VEREINFACHT_APP_URL", "https://app.verein.visuel.dev")
+
+      assert Mv.Config.vereinfacht_contact_view_url("abc") ==
+               "https://app.verein.visuel.dev/en/admin/finances/contacts/abc"
+    after
+      System.delete_env("VEREINFACHT_APP_URL")
+    end
   end
 
   defp set_vereinfacht_env(key, value) do