Fixed sidebar navigation crash when adding a second storefront

If you already had a site set up and opened Add storefront to connect a second one, every link in the dashboard's main sidebar (Instructions, Agents, Tools, Playground, Evaluations, Connectors, Settings, Reports) silently pointed at that "add storefront" page instead of your existing site. Clicking any of them crashed with an "Invalid site id" error.

This is now fixed: those links correctly fall back to your current site while you're on the "add storefront" page. The site switcher in the sidebar had the same underlying issue (it didn't crash, but it quietly disappeared on that page) and has been hardened the same way.