fix(platform): a folder dialog opens on a folder that exists (#331)

[chiarorosa]

What does this PR do?

Fixes #331. On a first run the new-project sheet's default location, <Desktop>/Concat, does not exist yet (default_location() only makes it on iOS), and platform::pick_folder passed it to rfd::FileDialog::set_directory as is. The dialog in the report is GNOME's chooser (Nautilus, behind the xdg portal): its "无法找到 "…/Desktop/Concat"" is Nautilus's Unable to find "%s". Please check the spelling and try again., a string only nautilus.mo carries, and the window then waits on that dialog until GNOME calls it unresponsive.

pick_folder now starts the dialog at the nearest folder of the path that exists, the desktop itself on a first run, and asks for no starting folder when none does (an empty start, as before). Export and Open call pick_folder the same way and get the same.

How to test

cargo test -p concat a_folder_dialog_starts_at_a_folder_that_exists. By hand, on GNOME 47+ with no ~/Desktop/Concat: New project › Location › Select opens on the desktop instead of an error.

What I ran, being on Windows without the native dependencies the whole workspace needs:

• rustfmt --check (1.93.1, the repo's rustfmt.toml) on platform.rs.
• The new function and test in a standalone crate: the test fails against the old behaviour (path passed through unchanged) and passes with the fix; cargo clippy --all-targets -- -D warnings is clean; it also fails under two mutations (only start itself checked, is_dir negated).
• In Docker (Debian/Ubuntu, Xvfb), rfd 0.17.2's pick_folder and GTK 4's chooser both open quietly elsewhere on a missing folder, so it's Nautilus specifically that errors; I could not run Nautilus's portal there, so the dialog in the report itself is not reproduced.

I have not built the concat crate itself, so this relies on CI for the workspace clippy and test.

Checklist

• cargo fmt --check passes (rustfmt on the changed file)
• cargo clippy --all-targets -- -D warnings passes (on the change in isolation; workspace left to CI)
• cargo test passes (the new test in isolation; workspace left to CI)
• python3 scripts/locales.py run (if UI text changed): no UI text changed
• New source files have a licence header: no new files