Restore Browser Preview Button

[petertzy]

Restore the pre-refactor feature that allowed users to click a button and automatically open a browser page showing the rendered Markdown content.

[Ruhamah-chioma]

I'd like to work on this. I'll dig into the git history to see how the feature worked before it was removed and get it working again in the current architecture — let me know if you have a preferred approach in mind.

[petertzy]

I've saved the complete code in the archive branch. Feel free to take your time reviewing it and working on this task. Thank you.