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/admin/theme_test.exs

@@ -29,7 +29,7 @@ defmodule BerrypodWeb.Admin.ThemeTest do
     test "renders theme editor page", %{conn: conn} do
       {:ok, _view, html} = live(conn, ~p"/admin/theme")
 
-      assert html =~ "Theme Studio"
+      assert html =~ "<h1 class=\"theme-title\">Theme</h1>"
       assert html =~ "preset"
     end