server.py and desktop_viewer.py read-only mode required the desktop
config JSONs to exist and crashed/showed "Config-Datei fehlt" if one
was deleted, even though the board already holds the config durably
in NVM. Add versapad_combined.fetch_from_board()/load_or_fetch(): a
missing combined JSON is now transparently rebuilt from the board via
serial and cached back to disk, falling back to the legacy per-profile
JSONs (or a clear error) only when the board is unreachable.
