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/live/admin/theme/index.ex

@@ -61,7 +61,12 @@ defmodule BerrypodWeb.Admin.Theme.Index do
         progress: &handle_progress/3
       )
 
-    {:ok, socket}
+    {:ok, assign(socket, :from_checklist, false)}
+  end
+
+  @impl true
+  def handle_params(params, _uri, socket) do
+    {:noreply, assign(socket, :from_checklist, params["from"] == "checklist")}
   end
 
   defp handle_progress(:logo_upload, entry, socket) do

lib/berrypod_web/live/admin/theme/index.html.heex

@@ -37,14 +37,21 @@
       <.link href={~p"/admin"} class="theme-back-link">
         <.icon name="hero-arrow-left-mini" class="size-4" /> Admin
       </.link>
+
+      <div :if={@from_checklist} class="admin-checklist-banner">
+        <.icon name="hero-clipboard-document-check" class="size-5 admin-checklist-banner-icon" />
+        <span class="admin-checklist-banner-text">
+          You're customising your theme.
+        </span>
+        <.link navigate={~p"/admin"} class="admin-link admin-checklist-banner-link">
+          &larr; Back to checklist
+        </.link>
+      </div>
       
 <!-- Header -->
       <div class="theme-header">
         <div class="admin-fill">
-          <h1 class="theme-title">Theme Studio</h1>
-          <p class="theme-subtitle">
-            One theme, infinite possibilities. Every combination is designed to work beautifully.
-          </p>
+          <h1 class="theme-title">Theme</h1>
         </div>
         <button
           type="button"