Allow choosing a custom recordings save location

[abduznik]

Problem

Recordings are saved under a fixed location under %AppData% on Windows (and the platform-equivalent elsewhere), with no way for the user to change it. Users who want their recordings on a different drive (more free space, a NAS mount, an SSD dedicated to captures, etc.) have no way to do that from within the app.

Proposed enhancement

Add a way to choose a custom folder for recordings to be saved to, instead of always defaulting to the app's data directory. Should include:

• A folder picker to select a custom location.
• A way to reset back to the default location.
• The choice should persist across app restarts.
• Should handle edge cases gracefully — e.g. what happens if the chosen folder becomes unavailable, or there's not enough disk space.

Resolved by

#650