Option to freeze screen while using area select

### Problem

In some cases it can be desirable to have the background freeze while drawing the area to take the screenshot from. Examples can include things that are moving/animated, something that could (potentially) vanish while you're drawing the area (like a notification that only lasts for a couple of seconds), frames from videos, ...

If the background would freeze up when entering area select mode, you would get a view of the exact screenshot you're going to take while drawing the rectangle, and you could potentially take time to make the area a bit cleaner instead of having to very quickly draw something before it's too late.

Essentially, this takes a snapshot of what your screen looks like when pressing the hotkey (complete screenshot), displays that, and lets you select an area from _that_ instead of from the live background.

### Proposal

A setting in the app that can be toggled on/off, off by default so people that are used to the current system don't have to notice the change.

### Prior Art (Optional)

- [Flameshot's area select](https://github.com/flameshot-org/flameshot#preview)
- [KDE Spectacle](https://github.com/KDE/spectacle)
- Windows 10 snipping tool (`win + shift + s`)

The issue names no files, tests, or entry points. Start by locating the existing area-select flow and screenshot capture path, then inspect how application settings are defined and applied. Done means an off-by-default setting freezes the screen on entry and allows selecting the intended area from that snapshot.

Written by the indexing model from the issue text.