Bring back post-sign-up route to the backend router (#64011)

Fixes [inc-313
](https://sourcegraph.slack.com/archives/C07D8ETFAQP/p1721746352871739?thread_ts=1721745191.614829&cid=C07D8ETFAQP)

It should solve problems with the old auth flow in the new Svelte
version. Currently, the go router doesn't know anything about
post-sign-up flow, which is used in the JB Cody extension. Because of
this, it passes it to the Svelte version which doesn't implement this
route on the client (but the react version did have it on the client
which is why it worked before, before we rolled out Svelte version by
default)

## Test plan
- There is no good way to check this properly, but after this PR you
have to see a react (old) version of the App when you go to the
/post-sigh-up router

cmd/frontend/internal/app/ui/router.go

@@ -148,6 +148,7 @@ func InitRouter(db database.DB) {
 		{path: "/password-reset", name: uirouter.RoutePasswordReset, title: "Reset password", index: false},
 		{path: "/survey", name: "survey", title: "Survey", index: false},
 		{path: "/survey/{score}", name: "survey-score", title: "Survey", index: false},
+		{path: "/post-sign-up", name: "post-sign-up", title: "Cody", index: false},
 	}
 
 	config := conf.Get()