OnionShare should warn when the user selects a non-writable directory

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Details about your operating system

Operating system: Linux Version/distribution: Debian Trixie, or Tails 7.4.2

Version of Onionshare: 2.6.3

How did you install OnionShare

• Native Mac/Windows package
• Flatpak
• Snap
• Pip (command line)
• Via my Linux distribution's package manager
• From source

What is the problem

When I open "Receive Files", I can select a non-writable directory (ie: /root/). OnionShare doesn't show any error, and allows me to start Receive Mode. Of course, receiving files will fail whenever files are actually received.

Now, you could think that selecting /root/ is not what any real user would do, but there are some cases in which this is less surprising. At Tails, we restrict what directories OnionShare has access to (using AppArmor), and limit it to Downloads/, which means that if a user selects another directory, they will suffer from this bug. See https://gitlab.tails.boum.org/tails/tails/-/issues/21425