Crash when opening a deleted folder from custom pools on the content browser

### Crash when opening a deleted folder from custom pools on the content browser
If a custom pool points to a deleted folder, opening it in the content browser causes Vortex to crash.

### Steps to reproduce
1. Open the content browser and create a pool for a folder.
2. Close Vortex and delete the folder.
3. Reopen Vortex with the same project and open the custom pool.
4. Vortex crashes.

### Environment
- OS: Arch Linux
- Version: all

### Expected
Implement a feature to invalidate dead custom pools, highlight them in red, and suggest deleting them. Also prevent crashes caused by deleted folders.

<img width="457" height="433" alt="Image" src="https://github.com/user-attachments/assets/633ed7cd-1e3c-44d9-85c1-09f2ed293359" />

backtrace :

<img width="2037" height="761" alt="Image" src="https://github.com/user-attachments/assets/ce19384a-2d6f-4d44-b133-e9d8e3b0a4f3" />

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Crash when opening a deleted folder from custom pools on the content browser #2