Tray menu option to switch the post-processing prompt

[mameli]

Handy already lets me switch the transcription model straight from the tray/menu-bar menu. I want to add a similar menu option because I use post-processing a lot and keep several prompts for different tasks, but switching between them means opening Handy → Settings → Post Process and picking from the dropdown each time.

Would others find it useful to have a post-processing prompt submenu in the tray, right below the model submenu — showing the selected prompt and letting me switch it without leaving whatever app I'm in? It would only appear when post-processing is enabled and would stay hidden while recording.

I've prototyped this locally and it works well, so I'd be happy to open a PR if there's interest.