Configurable Default Landing Page (Dashboard ⇆ Storefront)

Add a tenant-level toggle (Admin → Settings → Authentication) that lets admins choose the default page shown after customer login: Dashboard (current default) or Storefront/Webshop. On successful sign-in the platform should redirect server-side to the selected page; if the user lacks storefront access, gracefully fall back to the dashboard.

Requirements
Setting persists per tenant and applies at every login.
No intermediate flash of the dashboard when Storefront is chosen.
Existing nav links to both areas remain.

Benefits
Fewer clicks for e-commerce-focused tenants, faster purchase flow, higher conversion, while analytics-oriented tenants can keep the dashboard view.

Impact & Priority
Medium effort: tweak auth redirect logic, add a toggle in settings UI,