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>

test/berrypod_web/live/setup/onboarding_test.exs

@@ -69,7 +69,13 @@ defmodule BerrypodWeb.Setup.OnboardingTest do
       {:ok, view, _html} = live(conn, ~p"/setup")
 
       view
-      |> form(~s(form[phx-submit="create_account"]), account: %{email: "admin@example.com"})
+      |> form(~s(form[phx-submit="create_account"]),
+        account: %{
+          shop_name: "Test Shop",
+          email: "admin@example.com",
+          password: "valid_password_123"
+        }
+      )
       |> render_submit()
 
       # The LiveView redirects to /setup/login/:token