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/admin_layout_hook.ex

@@ -28,7 +28,6 @@ defmodule BerrypodWeb.AdminLayoutHook do
       socket
       |> assign(:current_path, "")
       |> assign(:site_live, Settings.site_live?())
-      |> assign(:email_configured, Berrypod.Mailer.email_configured?())
       |> assign(:theme_settings, theme_settings)
       |> assign(:site_name, Settings.site_name())
       |> assign(:site_description, Settings.site_description())