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/components/layouts/admin.html.heex

@@ -18,17 +18,6 @@
       </.link>
     </header>
 
-    <%!-- email warning banner --%>
-    <div :if={!@email_configured} class="admin-banner-warning">
-      <.icon name="hero-exclamation-triangle" class="size-5 shrink-0" />
-      <p>
-        Email delivery isn't set up yet — customers won't receive order confirmations or shipping updates.
-        <.link navigate={~p"/admin/settings/email"} class="admin-link">
-          Configure email
-        </.link>
-      </p>
-    </div>
-
     <%!-- page content --%>
     <main class="admin-main">
       <div class="admin-container">