The whole of a folder library's sign-in lived inside a Slint callback,
which cannot run without a display server — so the one path that decides
whether a mistyped folder becomes a *stored* account had no test at all.
That failure is quiet and lasting: an account for a directory that is
not there skips the launch screen on the next start and reads as a
library that has lost its photographs.
`open_folder_library` is that logic, lifted out whole. Four tests: it
stores an account with no credential and reaches the keyring for
nothing, a typo is refused before anything is written, the messages read
as instructions because they go straight to the screen's error line, and
a file is not a library.
dtourolle
64462e9fd3