fix: add fallback for vendors array in content page editor

Use (vendors || []) to prevent undefined iteration error in Alpine.js.

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

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

@@ -93,7 +93,7 @@
                         :disabled="loadingVendors"
                     >
                         <option :value="null">Platform Default</option>
-                        <template x-for="vendor in vendors" :key="vendor.id">
+                        <template x-for="vendor in (vendors || [])" :key="vendor.id">
                             <option :value="vendor.id" x-text="vendor.name"></option>
                         </template>
                     </select>