improve email settings progressive enhancement and admin layout

- semantic HTML: step numbers inside h2, strong provider names, details
  for adapter configs, strong error messages, fieldset drawer toggle hidden
- inline field errors via flash for no-JS controller fallback
- single form POST button for test email (works with and without JS)
- admin sidebar: remove brand/view-shop, move user email to footer nav
- replace inline style with .admin-setup-step-spaced class
- clean up unused CSS (.admin-brand, .admin-sidebar-header, etc.)

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

lib/berrypod_web/components/layouts/admin.html.heex

@@ -1,5 +1,5 @@
 <div class="admin-layout">
-  <input id="admin-drawer" type="checkbox" class="admin-layout-toggle" />
+  <input id="admin-drawer" type="checkbox" class="admin-layout-toggle" hidden />
 
   <%!-- main content area --%>
   <div class="admin-layout-content">
@@ -20,6 +20,7 @@
 
     <%!-- page content --%>
     <main class="admin-main">
+      <.flash_group flash={@flash} />
       <div class="admin-container">
         {@inner_content}
       </div>
@@ -30,23 +31,6 @@
   <div class="admin-sidebar-wrapper">
     <label for="admin-drawer" class="admin-sidebar-overlay" aria-label="Close navigation"></label>
     <aside class="admin-sidebar">
-      <%!-- sidebar header --%>
-      <div class="admin-sidebar-header">
-        <.link navigate={~p"/admin"} class="admin-brand">
-          Berrypod
-        </.link>
-        <p class="admin-text-secondary truncate">
-          {@current_scope.user.email}
-        </p>
-      </div>
-
-      <%!-- view shop link --%>
-      <div class="admin-sidebar-view-shop">
-        <.link href={~p"/"} class="admin-btn admin-btn-ghost admin-btn-sm admin-btn-block">
-          <.icon name="hero-arrow-top-right-on-square" class="size-4" /> View shop
-        </.link>
-      </div>
-
       <%!-- nav links --%>
       <nav class="admin-sidebar-nav" aria-label="Admin navigation">
         <div class="admin-nav-group">
@@ -189,6 +173,9 @@
       <%!-- sidebar footer --%>
       <div class="admin-sidebar-footer">
         <ul class="admin-nav">
+          <li class="admin-sidebar-email truncate">
+            <.icon name="hero-user" class="size-5" /> {@current_scope.user.email}
+          </li>
           <li>
             <details class="admin-dev-tools">
               <summary>
@@ -224,5 +211,3 @@
     </aside>
   </div>
 </div>
-
-<.flash_group flash={@flash} />