threads: Add topic tag to the dashboard composer

When creating or editing a Threads post in the Zernio dashboard, I can see the “Thread” option for chaining messages, but no field for the post’s topic tag. The API already supports platforms[].platformSpecificData.topic_tag: I used it successfully to publish a post tagged “Gaming Threads”.

Please add an optional topic tag field to the Threads composer, both when creating and editing a post, and show the selected tag in the post preview. This would let dashboard users set the topic without using an API key or a script.