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/credits.go

@@ -0,0 +1,8 @@
+package flow
+
+func (f *Flow) linkCredits() {
+	// Clicking anywhere in credits should return us
+	f.onClick(credits, "1", f.returnToLastDriver(credits))
+
+	// TODO: lots of text
+}