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>

assets/css/admin/components.css

@@ -1510,6 +1510,9 @@
 }
 
 .card-radio-name {
+  display: flex;
+  align-items: baseline;
+  gap: 0.25rem;
   font-size: 0.875rem;
   font-weight: 600;
 }
@@ -3391,26 +3394,21 @@
   margin-inline: auto;
 }
 
-/* Sidebar header (logo + user email) */
-.admin-sidebar-header {
-  padding: 1rem;
-  border-bottom: 1px solid var(--t-border-default);
-
-  & .admin-text-secondary { margin-top: 0.125rem; }
-}
-
-/* View shop link below sidebar header */
-.admin-sidebar-view-shop {
-  padding: 0.5rem;
-  border-bottom: 1px solid var(--t-border-default);
-}
-
 /* Sidebar footer (dev tools, log out) */
 .admin-sidebar-footer {
   padding: 0.5rem;
   border-top: 1px solid var(--t-border-default);
 }
 
+.admin-sidebar-email {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  padding: 0.5rem 0.75rem;
+  font-size: 0.75rem;
+  color: var(--t-text-secondary);
+}
+
 /* Dev tools disclosure in sidebar footer */
 .admin-dev-tools {
   & > summary {
@@ -3446,14 +3444,6 @@
   padding: 0.5rem;
 }
 
-/* Brand wordmark in sidebar */
-.admin-brand {
-  font-size: 1.125rem;
-  line-height: 1.75rem;
-  font-weight: 700;
-  letter-spacing: -0.025em;
-}
-
 /* Secondary text for user email, help text, meta info */
 .admin-text-secondary {
   font-size: 0.75rem;
@@ -4294,10 +4284,16 @@
   gap: 0.5rem;
 }
 
+.admin-setup-step-spaced { margin-top: 1.5rem; }
+
 .admin-setup-step-header {
   display: flex;
   align-items: center;
   gap: 0.625rem;
+  font-size: 0.9375rem;
+  font-weight: 600;
+  color: var(--admin-text-primary);
+  margin: 0;
 }
 
 .admin-setup-step-number {
@@ -4314,13 +4310,6 @@
   flex-shrink: 0;
 }
 
-.admin-setup-step-title {
-  font-size: 0.9375rem;
-  font-weight: 600;
-  color: var(--admin-text-primary);
-  margin: 0;
-}
-
 .admin-setup-step-desc {
   font-size: 0.8125rem;
   color: var(--admin-text-secondary);
@@ -4379,6 +4368,9 @@
   gap: 1.5rem;
 }
 
+/* Hide the <details> summary — CSS :has(:checked) controls visibility instead */
+.admin-adapter-config-summary { display: none; }
+
 /* Show only the adapter config matching the checked radio */
 .admin-adapter-config[data-adapter] { display: none; }