Setup wizard: use labeled-form grid for vertical field layout

Replace the fieldset-based layout with the app's standard
.labeled-form CSS grid so email, password and retype fields
stack vertically (label left, input right) exactly like the
existing sign-in and registration forms.

Section headings and checkbox rows are given explicit grid-column
spans via inline styles so they span the full form width rather
than being constrained to the label column.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/views/setup/new.html.erb

@@ -1,6 +1,8 @@
-<%= form_with url: setup_path, method: :post, class: "main-area" do |f| %>
-  <fieldset>
-    <legend><%= t(".admin_account") %></legend>
+<%= form_with url: setup_path, method: :post, class: "labeled-form main-area" do %>
+
+  <h3 style="grid-column: 1 / -1; text-align: left; margin: 0;">
+    <%= t(".admin_account") %>
+  </h3>
 
   <label for="admin_email"><%= t(".admin_email") %></label>
   <%= email_field_tag :admin_email, params[:admin_email],
@@ -16,23 +18,22 @@
   <%= password_field_tag :admin_password_confirmation, nil,
         id: "admin_password_confirmation", required: true, size: 30,
         autocomplete: "off" %>
-  </fieldset>
 
-  <fieldset>
-    <legend><%= t(".options") %></legend>
+  <h3 style="grid-column: 1 / -1; text-align: left; margin: 0.5em 0 0 0;">
+    <%= t(".options") %>
+  </h3>
 
-    <label for="skip_email_confirmation">
+  <label for="skip_email_confirmation" style="grid-column: 1 / 3; text-align: left;">
     <%= check_box_tag :skip_email_confirmation, "1",
           params[:skip_email_confirmation] == "1",
           id: "skip_email_confirmation" %>
     <%= t(".skip_email_confirmation") %>
   </label>
 
-    <label for="seed_units">
+  <label for="seed_units" style="grid-column: 1 / 3; text-align: left;">
     <%= check_box_tag :seed_units, "1", true, id: "seed_units" %>
     <%= t(".seed_units") %>
   </label>
-  </fieldset>
 
   <%= submit_tag t(".submit") %>
 <% end %>