`~/Projects` as default working directory

### Enhancement Summary

There's a new Free-Desktop standard (`XDG_PROJECTS_DIR`) that formalizes the `~/Projects` folder. I propose defaulting to that instead of `~/Documents`. Alternatively, only default to `Projects` if the folder already exists, and fallback to `Documents` otherwise

Related: #5869

### Implementation Details / Mockup

Same as current default, just using the old default as fallback. Ideally, also use the corresponding env-var according to the `xdg-user-dirs` specification.

### Please search the issue tracker for existing feature requests before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

Start by locating the current code that chooses the default working directory and review related issue #5869. Check how the project handles xdg-user-dirs and environment variables, then determine the fallback behavior between Projects and Documents. Done means the selected default follows the agreed precedence and works when either directory or the environment setting is absent.

Written by the indexing model from the issue text.