Feat: Refactor the MessageForm with shadcn #3221 (#7820)

### What problem does this PR solve?

Feat: Refactor the MessageForm with shadcn #3221

### Type of change


- [x] New Feature (non-breaking change which adds functionality)

web/src/pages/agent/form/begin-form/begin-dynamic-options.tsx

@@ -49,11 +49,7 @@ export function BeginDynamicOptions() {
           </div>
         );
       })}
-      <BlockButton
-        onClick={() => append({ value: '' })}
-        variant={'outline'}
-        type="button"
-      >
+      <BlockButton onClick={() => append({ value: '' })} type="button">
         {t('flow.addVariable')}
       </BlockButton>
     </div>