post_status_form: enable sync flush for watcher

This fixes drafts not clearing after posting a reply.

Vue 3.3.11 changed watchers to stop firing after component unmount.
After posting a reply, the post form is removed, now causing the queued
event to be discarded.
Synchronous flush causes the handler to be called immediately when
changes happen, solving the problem.

See: https://github.com/vuejs/core/pull/7181
See: 80e2128d52
Fixes: a7dea2f70f
Fixes: #413

src/components/post_status_form/post_status_form.js

@@ -329,6 +329,7 @@ const PostStatusForm = {
   watch: {
     'newStatus': {
       deep: true,
+      flush: 'sync',
       handler () {
         this.statusChanged()
       }