test: cover AprsmeWeb.Gettext helpers

- supported_locales/0 returns the declared ~w(en es de fr) list.
- put_locale/1 sets the Gettext process locale for each supported
  value and raises FunctionClauseError for unsupported input.

3 tests, async:false because they mutate process-wide Gettext state.

test/aprsme_web/gettext_test.exs

@@ -0,0 +1,33 @@
+defmodule AprsmeWeb.GettextTest do
+  use ExUnit.Case, async: false
+
+  alias AprsmeWeb.Gettext
+
+  setup do
+    original = Elixir.Gettext.get_locale(Gettext)
+    on_exit(fn -> Elixir.Gettext.put_locale(Gettext, original) end)
+    :ok
+  end
+
+  describe "supported_locales/0" do
+    test "returns the declared list" do
+      assert Gettext.supported_locales() == ~w(en es de fr)
+    end
+  end
+
+  describe "put_locale/1" do
+    test "sets the process-wide Gettext locale for supported values" do
+      Gettext.put_locale("es")
+      assert Elixir.Gettext.get_locale(Gettext) == "es"
+
+      Gettext.put_locale("fr")
+      assert Elixir.Gettext.get_locale(Gettext) == "fr"
+    end
+
+    test "raises FunctionClauseError for unsupported locales" do
+      assert_raise FunctionClauseError, fn ->
+        Gettext.put_locale("zz")
+      end
+    end
+  end
+end