rework email settings for true progressive enhancement

Render all adapter field sections in the form with CSS :has(:checked)
controlling visibility. Selecting a provider instantly shows its config
fields — no JS, no page reload, no server round-trip needed.

- Render all 6 adapter configs with data-adapter attribute
- CSS :has(:checked) show/hide rules per adapter in admin stylesheet
- Namespace field names per adapter (email[brevo][api_key] etc)
- Drop 4 transactional-only providers (Resend, Postmark, Mailgun, MailPace)
- Remove noscript "Switch provider" button and controller redirect workaround
- Remove configured_adapter hidden input tracking
- Hide JS-only test email button for no-JS users via noscript style
- LiveView progressively enhances with async save and test email

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

assets/css/admin/components.css

@@ -1492,7 +1492,7 @@
     outline-offset: 2px;
   }
 
-  &.card-radio-card-selected {
+  &:has(:checked) {
     border-color: var(--t-text-primary, #171717);
     background: var(--t-surface-sunken, #e5e5e5);
   }
@@ -4379,6 +4379,20 @@
   gap: 1.5rem;
 }
 
+/* Show only the adapter config matching the checked radio */
+.admin-adapter-config[data-adapter] { display: none; }
+
+.admin-content-medium:has(#email-adapter-brevo:checked) [data-adapter="brevo"],
+.admin-content-medium:has(#email-adapter-sendgrid:checked) [data-adapter="sendgrid"],
+.admin-content-medium:has(#email-adapter-mailjet:checked) [data-adapter="mailjet"],
+.admin-content-medium:has(#email-adapter-mailersend:checked) [data-adapter="mailersend"],
+.admin-content-medium:has(#email-adapter-smtp:checked) [data-adapter="smtp"],
+.admin-content-medium:has(#email-adapter-postal:checked) [data-adapter="postal"] {
+  display: flex;
+  flex-direction: column;
+  gap: 1.5rem;
+}
+
 /* ── Campaign form ── */
 
 .admin-campaign-actions {