fix(dialyzer): pass string module suffix to Igniter.Project.Module

Igniter.Project.Module.module_name/2 is specced as (Igniter.t(), String.t()).
Using the atom MainWindow broke Dialyzer (invalid call, no_return on igniter/1,
and spurious unused_fun on private helpers). Use "MainWindow" consistently.

Unblocks CI for #69

Co-authored-by: Dominic Letz <dominicletz@users.noreply.github.com>

Author: cursoragent

lib/mix/tasks/desktop.install.ex

@@ -50,7 +50,7 @@ if Code.ensure_loaded?(Igniter.Mix.Task) do
       menu = Igniter.Project.Module.module_name(igniter, "Menu")
       menubar = Igniter.Project.Module.module_name(igniter, "MenuBar")
       gettext = Igniter.Libs.Phoenix.web_module_name(igniter, "Gettext")
-      main_window = Igniter.Project.Module.module_name(igniter, MainWindow)
+      main_window = Igniter.Project.Module.module_name(igniter, "MainWindow")
 
       igniter
       |> Igniter.compose_task("igniter.add", ["desktop"])
@@ -63,7 +63,7 @@ if Code.ensure_loaded?(Igniter.Mix.Task) do
            quote do
              [
                app: unquote(app),
-               id: unquote(Igniter.Project.Module.module_name(igniter, MainWindow)),
+               id: unquote(Igniter.Project.Module.module_name(igniter, "MainWindow")),
                title: unquote(to_string(app)),
                size: {600, 500},
                menubar: unquote(menubar),