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/theme/fonts.ex

@@ -128,9 +128,7 @@ defmodule SimpleshopTheme.Theme.Fonts do
         [heading_key, body_key]
       end
 
-    font_keys
-    |> Enum.map(&generate_font_face_for_font(&1, path_resolver))
-    |> Enum.join("\n")
+    Enum.map_join(font_keys, "\n", &generate_font_face_for_font(&1, path_resolver))
   end
 
   @doc """
@@ -143,8 +141,7 @@ defmodule SimpleshopTheme.Theme.Fonts do
   def generate_all_font_faces(path_resolver \\ &default_path_resolver/1) do
     @fonts
     |> Map.keys()
-    |> Enum.map(&generate_font_face_for_font(&1, path_resolver))
-    |> Enum.join("\n")
+    |> Enum.map_join("\n", &generate_font_face_for_font(&1, path_resolver))
   end
 
   @doc """
@@ -213,8 +210,7 @@ defmodule SimpleshopTheme.Theme.Fonts do
   defp generate_font_face_for_font(key, path_resolver) do
     case get_font(key) do
       %{family: family, file_prefix: prefix, weights: weights} ->
-        weights
-        |> Enum.map(fn weight ->
+        Enum.map_join(weights, "", fn weight ->
           weight_suffix = if weight == 400, do: "regular", else: to_string(weight)
           font_path = path_resolver.("/fonts/#{prefix}-#{weight_suffix}.woff2")
 
@@ -228,7 +224,6 @@ defmodule SimpleshopTheme.Theme.Fonts do
           }
           """
         end)
-        |> Enum.join("")
 
       nil ->
         ""

lib/simpleshop_theme/theme/preview_data.ex

@@ -393,8 +393,7 @@ defmodule SimpleshopTheme.Theme.PreviewData do
     # Get available variants for pricing
     available_variants =
       product.variants
-      |> Enum.filter(& &1.is_enabled)
-      |> Enum.filter(& &1.is_available)
+      |> Enum.filter(&(&1.is_enabled and &1.is_available))
 
     # Get the cheapest available variant for display price
     cheapest_variant =