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/login.ex

@@ -12,17 +12,10 @@ defmodule BerrypodWeb.Auth.Login do
           <.header>
             <p>Log in</p>
             <:subtitle>
-              <%= cond do %>
-                <% @current_scope -> %>
-                  You need to reauthenticate to perform sensitive actions on your account.
-                <% @registration_open -> %>
-                  Don't have an account? <.link
-                    navigate={~p"/setup"}
-                    class="font-semibold text-brand hover:underline"
-                    phx-no-format
-                  >Set up your shop</.link> to get started.
-                <% true -> %>
-                  Log in with your admin credentials.
+              <%= if @current_scope do %>
+                You need to reauthenticate to perform sensitive actions on your account.
+              <% else %>
+                Log in with your admin credentials.
               <% end %>
             </:subtitle>
           </.header>
@@ -116,7 +109,6 @@ defmodule BerrypodWeb.Auth.Login do
      assign(socket,
        form: form,
        trigger_submit: false,
-       registration_open: !Accounts.has_admin?(),
        email_configured: Mailer.email_verified?()
      )}
   end

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