Hide the synthetic Default Title option on single-SKU products

Shopify gives every single-variant product one option named Title with the
lone value Default Title. It isn't a real choice, so filter it out of the
product detail options, the cart line summaries, and the payload the store
assistant sees. Variant matching still reads the raw selectedOptions, so
single-SKU products keep resolving their variant.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EuRVxuyMUTzWKXW8P5Tn8A

services/shopify/catalog.ts

@@ -64,6 +64,25 @@ export interface ProductOption {
   optionValues?: ProductOptionValue[];
 }
 
+/**
+ * Single-variant products still carry one synthetic option — `Title` with the
+ * lone value `Default Title`. It isn't a real choice, so keep it out of the UI.
+ */
+export const isDefaultTitleOption = (option: {
+  name: string;
+  values: string[];
+}): boolean =>
+  option.name === 'Title' &&
+  option.values.length === 1 &&
+  option.values[0] === 'Default Title';
+
+/** Same synthetic option, as it appears on a variant's `selectedOptions`. */
+export const isDefaultTitleSelection = (selection: {
+  name: string;
+  value: string;
+}): boolean =>
+  selection.name === 'Title' && selection.value === 'Default Title';
+
 export interface Product {
   id: string;
   title: string;