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

lib/mv/config.ex

@@ -178,6 +178,37 @@ defmodule Mv.Config do
     env_or_setting("VEREINFACHT_CLUB_ID", :vereinfacht_club_id)
   end
 
+  @doc """
+  Returns the Vereinfacht app base URL for contact view links (frontend, not API).
+
+  Reads from `VEREINFACHT_APP_URL` env first, then from Settings.
+  Used to build links like https://app.verein.visuel.dev/en/admin/finances/contacts/{id}.
+  If not set, derived from API URL by replacing host \"api.\" with \"app.\" when possible.
+  """
+  @spec vereinfacht_app_url() :: String.t() | nil
+  def vereinfacht_app_url do
+    env_or_setting("VEREINFACHT_APP_URL", :vereinfacht_app_url) ||
+      derive_app_url_from_api_url(vereinfacht_api_url())
+  end
+
+  defp derive_app_url_from_api_url(nil), do: nil
+
+  defp derive_app_url_from_api_url(api_url) when is_binary(api_url) do
+    api_url = String.trim(api_url)
+    uri = URI.parse(api_url)
+    host = uri.host || ""
+
+    if String.starts_with?(host, "api.") do
+      app_host = "app." <> String.slice(host, 4..-1//1)
+      scheme = uri.scheme || "https"
+      "#{scheme}://#{app_host}"
+    else
+      nil
+    end
+  end
+
+  defp derive_app_url_from_api_url(_), do: nil
+
   @doc """
   Returns true if Vereinfacht is fully configured (URL, API key, and club ID all set).
   """
@@ -211,6 +242,11 @@ defmodule Mv.Config do
   """
   def vereinfacht_club_id_env_set?, do: env_set?("VEREINFACHT_CLUB_ID")
 
+  @doc """
+  Returns true if VEREINFACHT_APP_URL is set (field is read-only in Settings).
+  """
+  def vereinfacht_app_url_env_set?, do: env_set?("VEREINFACHT_APP_URL")
+
   defp env_set?(key) do
     case System.get_env(key) do
       nil -> false
@@ -241,18 +277,22 @@ defmodule Mv.Config do
   end
 
   @doc """
-  Returns the URL to view a finance contact (e.g. in Vereinfacht frontend or API).
+  Returns the URL to view a finance contact in the Vereinfacht app (frontend).
 
-  Uses the configured API base URL and appends /finance-contacts/{id}.
-  Can be extended later with a dedicated frontend URL setting.
+  Uses the configured app base URL (or derived from API URL) and appends
+  /en/admin/finances/contacts/{id}. Returns nil if no app URL can be determined.
   """
   @spec vereinfacht_contact_view_url(String.t()) :: String.t() | nil
   def vereinfacht_contact_view_url(contact_id) when is_binary(contact_id) do
-    base = vereinfacht_api_url()
+    base = vereinfacht_app_url()
 
-    if present?(base),
-      do: base |> String.trim_trailing("/") |> then(&"#{&1}/finance-contacts/#{contact_id}"),
-      else: nil
+    if present?(base) do
+      base
+      |> String.trim_trailing("/")
+      |> then(&"#{&1}/en/admin/finances/contacts/#{contact_id}")
+    else
+      nil
+    end
   end
 
   defp present?(nil), do: false