add CI pipeline with credo and dialyzer

mix ci alias: compile --warning-as-errors, format --check-formatted,
credo, dialyzer, test. Credo configured with sensible defaults.
Dialyzer ignore file for false positives (Stripe types, Mix tasks,
ExUnit internals). Credo fixes: map_join, filter consolidation,
nesting extraction, cond→if simplification.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/simpleshop_theme_web/controllers/checkout_controller.ex

@@ -10,14 +10,12 @@ defmodule SimpleshopThemeWeb.CheckoutController do
     cart_items = Cart.get_from_session(get_session(conn))
     hydrated = Cart.hydrate(cart_items)
 
-    cond do
-      hydrated == [] ->
-        conn
-        |> put_flash(:error, "Your basket is empty")
-        |> redirect(to: ~p"/cart")
-
-      true ->
-        create_checkout(conn, hydrated)
+    if hydrated == [] do
+      conn
+      |> put_flash(:error, "Your basket is empty")
+      |> redirect(to: ~p"/cart")
+    else
+      create_checkout(conn, hydrated)
     end
   end