Server launcher and start-up checks for other machines

- server/run_server.sh: checks Godot is 4.7+, imports a fresh copy once
  (the class cache in .godot/ isn't in git), then starts the server
- The dedicated server refuses to start, with a clear reason, on Godot
  older than 4.7 or when the game's scripts failed to load, instead of
  claiming to listen
- tests/run_all.sh imports a fresh copy first
- Setup doc: 4.7+ (not 4.3+), importing a fresh copy, running a server

scripts/net/net.gd

@@ -144,7 +144,26 @@ func is_server() -> bool:
 
 # --- server -------------------------------------------------------------------
 
+## Why this Godot can't run the server ("" = it can): too old, or the game's
+## scripts failed to load (usually a fresh copy that's never been imported).
+func startup_problem() -> String:
+	var v := Engine.get_version_info()
+	if v.major < 4 or (v.major == 4 and v.minor < 7):
+		return "This game needs Godot 4.7 or newer, but this is %s. Download 4.7.2 from https://godotengine.org/download" % v.string
+	var root: Window = (Engine.get_main_loop() as SceneTree).root
+	for n in ["ItemDB", "Actions", "Saves"]:
+		var node := root.get_node_or_null(n)
+		if node == null or node.get_script() == null:
+			return "The game's scripts didn't load (see the errors above). If this is a fresh copy of the project, " + \
+				"import it once first:  godot --headless --path . --import   (server/run_server.sh does this for you)"
+	return ""
+
 func _start_dedicated() -> void:
+	var bad := startup_problem()
+	if not bad.is_empty():
+		printerr("[server] Not starting: ", bad)
+		get_tree().quit(1)
+		return
 	var err := start_server(String(_args.get("config", ServerConfig.DEFAULT_PATH)), _args)
 	if not err.is_empty():
 		printerr("[server] ", err)