Wire up inventory select to ship state

Also mixed into this commit:

* Use returning drivers where possible
* Make the credits screen returnable via click

internal/flow/load_game.go

@@ -2,5 +2,5 @@ package flow
 
 func (f *Flow) linkLoadGame() {
 	// Load game
-	f.onClick(loadGame, "3.3", f.setDriver(main)) // Cancel button
+	f.onClick(loadGame, "3.3", f.returnToLastDriver(loadGame)) // Cancel button
 }