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/setup_hook.ex

@@ -0,0 +1,18 @@
+defmodule BerrypodWeb.SetupHook do
+  @moduledoc """
+  Redirects to /setup when no admin account exists (fresh install).
+
+  Add `{BerrypodWeb.SetupHook, :require_admin}` to any live_session
+  that shouldn't be accessible before setup is complete.
+  """
+
+  import Phoenix.LiveView, only: [redirect: 2]
+
+  def on_mount(:require_admin, _params, _session, socket) do
+    if Berrypod.Accounts.has_admin?() do
+      {:cont, socket}
+    else
+      {:halt, redirect(socket, to: "/setup")}
+    end
+  end
+end