fix: report the real error when a Windows G-code export fails
copy_file built its failure message as "Error: " + errCode. Adding a DWORD
to a string literal is pointer arithmetic, not concatenation, so the pointer
lands errCode bytes into an 8-byte literal and runs past its end for any code
above 7. std::string then calls strlen on it and throws length_error, and the
catch(...) in BackgroundSlicingProcess::finalize_gcode replaces the diagnosis
with "Unknown error occurred during exporting G-code."
Every code a user is likely to hit is past the end: write-protected media is
19, no media 21, a full disk 112, and a destination held open by another
program 32. Codes 1 to 7 stay inside the literal and produce a truncated
message instead. So the "Maybe the SD card is write locked?" text has not
been reachable on Windows since this path was added in #2923.
Now that it is reachable, that guess only fits removable media, so it is
conditional on m_export_path_on_removable_media. The existing string is
untouched and keeps its 23 translations; the fixed-drive case adds one string.
On Linux every account shares /tmp, but slicing builds temp paths there under
fixed, app-owned names via temporary_dir() (model backups, STEP import,
part-skip). The first user to slice creates and owns those dirs, so the next
user cannot write under them and slicing crashes with "No such file or
directory".
Tag the app temp root with the user id at startup (<temp>/orcaslicer_<uid>)
so every temporary_dir() consumer is isolated at once. The id stays at the
top level of the world-writable system temp so each user's dir is created
directly there; a shared parent dir would be owned by whichever user made it
first. The root is pre-created because STEP import writes into it directly.
Windows keeps the plain temp dir since it is already per-user.
Fixes#10108. Same root cause as #5969.
Kris Austin