improve setup UX: password field, setup hook, checklist banners, theme tweaks

- add password field and required shop name to setup wizard
- extract SetupHook for DRY redirect to /setup when no admin exists
- add ?from=checklist param to checklist hrefs with contextual banner on
  email settings and theme pages for easy return to dashboard
- remove email warning banner from admin layout (checklist covers it)
- make email a required checklist item (no longer optional)
- add DevReset module for wiping dev data without restart
- rename "Theme Studio" to "Theme", drop subtitle
- lower theme editor side-by-side breakpoint from 64em to 48em
- clean up login/registration pages (remove dead registration_open code)
- fix settings.put_secret to invalidate cache after write

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod_web/live/auth/registration.ex

@@ -48,15 +48,11 @@ defmodule BerrypodWeb.Auth.Registration do
   end
 
   def mount(_params, _session, socket) do
-    if Accounts.has_admin?() do
-      {:ok,
-       socket
-       |> put_flash(:error, "Registration is closed")
-       |> redirect(to: ~p"/users/log-in")}
-    else
-      # Fresh install — account creation happens on the setup page
-      {:ok, redirect(socket, to: ~p"/setup")}
-    end
+    # Admin exists (hook handles no-admin), registration is single-user only
+    {:ok,
+     socket
+     |> put_flash(:error, "Registration is closed")
+     |> redirect(to: ~p"/users/log-in")}
   end
 
   @impl true