fix(core,fuse,cli,gui): refuse to mount My files over a folder with files in it

FUSE mounts over a non-empty folder without complaint, so files already
in the My files folder were hidden under the mount: not uploaded, and
back only once the service stopped, so they looked deleted (#67, B277).

- pdfs-core: new `mountpoint` module. `not_empty` lists what is in a
  folder (hidden entries count); `mounted_fs_type` moves here from the
  CLI so the daemon can use it. `MountFailure` gains an optional
  `not_empty` field, which older readers ignore.
- pdfs-fuse: `mount_with` checks the folder before the root fetch and
  before any worker starts. A dead mount is cleared first; one of our
  own live mounts is left to `spawn_session`, as before.
- pdfs-cli: `cmd_daemon` records the folder in the failure record. The
  refusal backs off and retries like any mount failure, so emptying the
  folder is enough.
- pdfs-gui: the status pages explain the refusal, and a dialog offers
  to open the folder or choose another one, once per run of failures.
  The folder picker refuses a folder with files in it.

Translated in all catalogs. Docs: BUGS (B277), CHANGELOG, FAQ,
TROUBLESHOOTING, CONFIGURATION.

docs/TROUBLESHOOTING.md

@@ -141,6 +141,20 @@ fusermount3 -uz ~/ProtonDrive
 systemctl --user restart proton-drive.service
 ```
 
+### My files does not open: the folder is not empty
+
+The app says **Your Proton Drive Folder Isn't Empty**, and `pdfs diagnose` reports
+`mount: failing … is not empty`. The folder chosen for My files has files in it, and the service
+does not open My files over them: they would be hidden, and not uploaded, for as long as the
+service runs.
+
+Move the files out of the folder, or choose another folder in **Preferences → General →
+Location**. The service tries again on its own and opens once the folder is empty. To put the
+files on Proton Drive, copy them into My files once it is open.
+
+Files saved into `~/ProtonDrive` while the service was stopped are in the same place: in the
+local folder, not on Proton Drive.
+
 ### The folder is empty or missing
 
 Check that the service runs and is signed in, with `pdfs status`. The service waits for a stored