fix: resolve content page editor JS errors and vendor dropdown

- Escape hyphen in regex pattern to fix Firefox regexp validation error
- Fix vendor API response parsing (use data.vendors instead of data.items)

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

app/templates/admin/content-page-edit.html

@@ -73,7 +73,7 @@
                         x-model="form.slug"
                         required
                         maxlength="100"
-                        pattern="[a-z0-9_-]+"
+                        pattern="[a-z0-9\-_]+"
                         class="w-full px-3 py-2 text-gray-700 dark:text-gray-300 border border-gray-300 dark:border-gray-600 rounded-lg focus:outline-none focus:border-purple-500 dark:bg-gray-700"
                         placeholder="about"
                     >