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/mockups/generator.ex

@@ -214,7 +214,7 @@ defmodule SimpleshopTheme.Mockups.Generator do
   """
   def find_print_provider(blueprint_id) do
     case Client.get_print_providers(blueprint_id) do
-      {:ok, providers} when is_list(providers) and length(providers) > 0 ->
+      {:ok, providers} when is_list(providers) and providers != [] ->
         # Just pick the first provider for simplicity
         {:ok, hd(providers)}