fix(locale): guard LiveView mounts against legacy nil session locales

The pre-localize PutSession plug wrote the CLDR tag's gettext_locale_name into the session verbatim, which could be nil.
Such a session matched the on_mount pattern and Gettext.put_locale/2 raises on nil, crashing every LiveView mount for that browser until
the cookie is replaced. Only accept binary locale values; anything else falls back to the Gettext default.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: JakobLichterfeld

lib/teslamate_web/live/init_assigns.ex

@@ -6,9 +6,15 @@ defmodule TeslaMateWeb.InitAssigns do
   import Phoenix.Component
 
   def on_mount(:locale, _params, session, socket) do
+    # is_binary: the pre-localize PutSession plug wrote the CLDR tag's
+    # gettext_locale_name verbatim, which could be nil — and
+    # Gettext.put_locale/2 raises on nil.
     case session do
-      %{"gettext_locale" => locale} -> Gettext.put_locale(TeslaMateWeb.Gettext, locale)
-      _other -> :ok
+      %{"gettext_locale" => locale} when is_binary(locale) ->
+        Gettext.put_locale(TeslaMateWeb.Gettext, locale)
+
+      _other ->
+        :ok
     end
 
     {:cont, assign(socket, :locale, Gettext.get_locale(TeslaMateWeb.Gettext))}

test/teslamate_web/locale_test.exs

@@ -128,6 +128,18 @@ defmodule TeslaMateWeb.LocaleTest do
     assert html =~ "Einstellungen"
   end
 
+  test "LiveView mounts ignore legacy nil session locales" do
+    assert {:cont, socket} =
+             TeslaMateWeb.InitAssigns.on_mount(
+               :locale,
+               %{},
+               %{"gettext_locale" => nil},
+               %Phoenix.LiveView.Socket{}
+             )
+
+    assert socket.assigns.locale == "en"
+  end
+
   test "supported_locales config stays in sync with the Gettext locales" do
     configured =
       :localize