feat: add product sorting to collection pages with tests

Add sort functionality to /collections/:slug pages with 6 sort options
(featured, newest, price ascending/descending, name A-Z/Z-A). Sort
selection persists across category navigation via URL query params.

Refactor handle_params to be DRY using load_collection/1 helper.
Add comprehensive unit tests for PreviewData category functions and
LiveView tests for the Collection page sorting and navigation.

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

test/simpleshop_theme/theme/preview_data_test.exs

@@ -198,4 +198,81 @@ defmodule SimpleshopTheme.Theme.PreviewDataTest do
       assert PreviewData.has_real_products?() == false
     end
   end
+
+  describe "category_by_slug/1" do
+    test "returns category when slug exists" do
+      category = PreviewData.category_by_slug("art-prints")
+
+      assert category != nil
+      assert category.slug == "art-prints"
+      assert is_binary(category.name)
+    end
+
+    test "returns nil when slug does not exist" do
+      assert PreviewData.category_by_slug("nonexistent") == nil
+    end
+
+    test "finds all known categories by slug" do
+      categories = PreviewData.categories()
+
+      for category <- categories do
+        found = PreviewData.category_by_slug(category.slug)
+        assert found != nil
+        assert found.id == category.id
+      end
+    end
+  end
+
+  describe "products_by_category/1" do
+    test "returns all products for nil" do
+      all_products = PreviewData.products()
+      filtered = PreviewData.products_by_category(nil)
+
+      assert filtered == all_products
+    end
+
+    test "returns all products for 'all'" do
+      all_products = PreviewData.products()
+      filtered = PreviewData.products_by_category("all")
+
+      assert filtered == all_products
+    end
+
+    test "returns empty list for nonexistent category" do
+      assert PreviewData.products_by_category("nonexistent") == []
+    end
+
+    test "returns only products matching the category" do
+      category = List.first(PreviewData.categories())
+      products = PreviewData.products_by_category(category.slug)
+
+      assert is_list(products)
+
+      for product <- products do
+        assert product.category == category.name
+      end
+    end
+
+    test "products are filtered correctly for each category" do
+      categories = PreviewData.categories()
+
+      for category <- categories do
+        products = PreviewData.products_by_category(category.slug)
+
+        for product <- products do
+          assert product.category == category.name
+        end
+      end
+    end
+
+    test "all products belong to at least one category" do
+      all_products = PreviewData.products()
+      categories = PreviewData.categories()
+      category_names = Enum.map(categories, & &1.name)
+
+      for product <- all_products do
+        assert product.category in category_names
+      end
+    end
+  end
 end