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/components/shop_components/content.ex

@@ -1199,7 +1199,6 @@ defmodule SimpleshopThemeWeb.ShopComponents.Content do
 
     widths
     |> Enum.sort()
-    |> Enum.map(&"#{base}#{separator}#{&1}.#{format} #{&1}w")
-    |> Enum.join(", ")
+    |> Enum.map_join(", ", &"#{base}#{separator}#{&1}.#{format} #{&1}w")
   end
 end

lib/simpleshop_theme_web/components/shop_components/product.ex

@@ -417,8 +417,7 @@ defmodule SimpleshopThemeWeb.ShopComponents.Product do
     gap_class = gap || ""
 
     [base, cols, gap_class, extra_class]
-    |> Enum.reject(&is_nil/1)
-    |> Enum.reject(&(&1 == ""))
+    |> Enum.reject(&(is_nil(&1) or &1 == ""))
     |> Enum.join(" ")
   end