/user?tab=count. That means it should rather be a hyperlink where your application handles the routing and provides state to identify the active tab.
JSON.stringifyback and forth. It can be safely passed between the client and the server, localStorage or to web workers. You will use strings, numbers, booleans, arrays, objects and null. Overmind also has the ability to allow you define state values as class instances, even serializing back and forth. You can read more about that in State.