Instead of giving user presets their parent's 2.4.2 printers in memory,
detect the user presets that inherit their parent's compatible_printers
and are missing printers from the legacy map, and offer to migrate them
at startup or from Help > Migrate Filament Presets.
Migrating first backs up the user folder to
filament_preset_migration_<datetime>_backup.zip in the configuration
folder, and migrates nothing if that fails. It then writes the printers
into each preset's compatible_printers, patching only that key so the
preset keeps the parent name it was saved with and older releases can
still resolve it. Cloud-synced presets are marked for upload.
No asks again at the next launch; "Don't remind me again" stops the
startup prompt.