Warn (or disallow) when using reserved filesystem paths

[devinrsmith]

It would be nice if the flatseal UI was able to give the user a warning (or disallow) when using a reserved filesystem paths. As a new user to flatpak / flatseal, it would be very helpful.

I needed some advice originally (#709 (comment)) to diagnose why my path was not showing up:

$ flatpak run --command=sh com.jetbrains.IntelliJ-IDEA-Community
F: Not sharing "/usr/lib/jvm/java-21-zulu-openjdk-ca" with sandbox: Path "/usr" is reserved by Flatpak
[📦 com.jetbrains.IntelliJ-IDEA-Community ~]$ 
exit