Lock-Symbol for read-only panes

Nobody has claimed this yet.

# Description of the new feature/enhancement

Find a way to highlight the fact, that a pane is in read-only mode

## Why though?

I use a 3 pane setup when I'm developing a web app.

1) just a console, git and stuff
2) watch-build backend
3) watch-build frontend

I want to be 2) and 3) in read-only mode most of the time to avoid typing into the pane when it accidentally gets focus. I dont know why but that happens quite often to me. But sometimes I want to cancel the process and then I never know if the pane is in read-only mode. That's kinda annoying

# Proposed technical implementation details (optional)

Some ideas:
- show a lock icon in the upper right corner of the pane for example
- change the background color of the pane
- change the frame color of the pane

A lock symbol in the tab is not good enough because it doesn't imply which pane is / which panes are locked if you have more than one pane.

The issue names no files, tests, or entry points; start by locating the pane read-only state and the existing pane or tab UI in the Windows Terminal codebase. Before implementation, settle whether the indicator is an icon, background, or frame change, then verify that it distinguishes each locked pane rather than relying on a tab-level marker.

Written by the indexing model from the issue text.