Add visual indicator when theme saved via +list-themes

[rewdy]

What do you want to change?

I want to update the +list-themes command so that when a user types w to save their theme, a small visual indicator it shown to confirm that something was done.

Why do you want to make this change?

I have been using ghostty for a bit now and found it confusing that when I typed w to save my theme, there was no indication anything happened. It made me wonder if it worked properly or failed. It's a small, isolated task that I thought could be a good first contribution.

I acknowledge that:

• I have read the Contributing Guide and understand the contribution process.
• I have read and agree to follow the AI Usage Policy.
• I wrote this vouch request myself, in my own voice, without AI generating it.

[jcollie]

!vouch