fix: Product form TypeScript and API errors

Fixed Issues:
1. TypeScript error on .indeterminate property (line 332)
   - Cast checkbox element to any for indeterminate access
2. API error handling for categories/tags endpoints
   - Added is_wp_error() checks
   - Return empty array on error instead of 500

Next: Implement modern tabbed product form (Shopify-style)

admin-spa/src/routes/Products/index.tsx

@@ -329,7 +329,11 @@ export default function Products() {
                 <th className="w-12 p-3">
                   <Checkbox
                     checked={allSelected}
-                    ref={(el) => el && (el.indeterminate = someSelected && !allSelected)}
+                    ref={(el) => {
+                      if (el) {
+                        (el as any).indeterminate = someSelected && !allSelected;
+                      }
+                    }}
                     onCheckedChange={toggleAll}
                     aria-label={__('Select all')}
                   />