test: add settings → CSS cache invalidation test

Verifies that updating theme settings regenerates the CSS cache
with the new values. Tests accent color changes from red to blue
and confirms the correct HSL hue values appear in cached CSS.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

test/simpleshop_theme/settings_test.exs

@@ -63,6 +63,31 @@ defmodule SimpleshopTheme.SettingsTest do
       assert settings.typography == "editorial"
     end
 
+    test "regenerates CSS cache when settings change" do
+      alias SimpleshopTheme.Theme.CSSCache
+
+      # Get initial cached CSS
+      {:ok, initial_css} = CSSCache.get()
+      assert initial_css =~ ".themed {"
+
+      # Update to a different accent color
+      {:ok, _settings} = Settings.update_theme_settings(%{accent_color: "#ff0000"})
+
+      # Cache should now contain new CSS with the red accent color
+      {:ok, updated_css} = CSSCache.get()
+      assert updated_css =~ ".themed {"
+      assert updated_css =~ "--t-accent-h: 0"  # Red = hue 0
+
+      # Change to blue
+      {:ok, _settings} = Settings.update_theme_settings(%{accent_color: "#0000ff"})
+
+      {:ok, blue_css} = CSSCache.get()
+      assert blue_css =~ "--t-accent-h: 240"  # Blue = hue 240
+
+      # Restore default
+      CSSCache.warm()
+    end
+
     test "validates mood values" do
       {:error, changeset} = Settings.update_theme_settings(%{mood: "invalid"})
       assert "is invalid" in errors_on(changeset).mood