fix: resolve tiptap duplicate Link extension warning

StarterKit 3.10+ now includes Link by default. Our code was adding
Link.configure() separately, causing duplicate extension warning and
breaking the email builder visual editor modal.

Fixed by configuring StarterKit with { link: false } so our custom
Link.configure() with specific options is the only Link extension.

admin-spa/src/components/ui/rich-text-editor.tsx

@@ -45,7 +45,11 @@ export function RichTextEditor({
 }: RichTextEditorProps) {
   const editor = useEditor({
     extensions: [
-      StarterKit,
+      // StarterKit 3.10+ includes Link by default, so disable it here
+      // since we configure Link separately below with custom options
+      StarterKit.configure({
+        link: false,
+      }),
       Placeholder.configure({
         placeholder,
       }),