fix: validate custom folders before launch

dzgui/strings/offline.py

@@ -13,6 +13,7 @@ local_frame = "Installed mods"
 no_local_mods = "No local mods found"
 no_mods = "No valid mods found"
 no_mission = "Not a valid mission"
+folder_changed = "Folder missing or changed on disk"
 
 custom_eventbox = "Set the root folder. Mods must be in atomic folders and have a meta.cpp file at a minimum."
 mission_eventbox = "Select a single mission folder containing an init.c file."