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>

docs/plans/onboarding-ux.md

@@ -1,6 +1,6 @@
 # Onboarding UX v2
 
-Status: Planned
+Status: In progress
 
 Supersedes the original onboarding-ux plan. Based on usability testing session (March 2026) covering the setup wizard, launch checklist, email provider setup, and general onboarding flow.
 
@@ -77,20 +77,31 @@ API key entry should be as forgiving as possible:
 
 **Files:** `lib/berrypod_web/live/setup/onboarding.ex`, `lib/berrypod_web/live/admin/providers.ex`, validation logic in contexts
 
-### D. Email provider setup UX
+### D. Email provider setup UX — done
 
-Rework the email provider selection and configuration:
+Reworked the email provider selection and configuration. Changes across two sessions:
 
-- **Recommended pick** highlighted at top — whichever has the best free tier, easiest setup, and supports newsletters. One clear "if you're not sure, pick this" option.
-- **Two groups:** "Also sends newsletters" vs "Transactional only" with one-line explanation of the difference
-- **One-liner per provider:** free tier info, setup difficulty ("paste one API key" vs "requires domain verification")
-- **Self-hosted/SMTP** in a collapsible "Advanced" section at bottom
-- **Guided flow after selection:** link to create account (new tab), then "Now enter these settings:" with config fields
-- **Send test email** button after configuration
-- **Remove** the masked key display ("Current: 84159e26•••4f3")
-- **Default from address** to the admin email automatically — don't surface during setup, it's a general settings thing for later
+**Session 1 (commit dd659e4):**
+- Recommended pick (Brevo) highlighted with badge
+- Two groups: "Also sends newsletters" vs "Transactional only"
+- One-liner per provider with free tier info
+- Self-hosted/SMTP in collapsible "Advanced" section
+- Guided flow after selection with config fields
+- Send test email button with async delivery
+- Removed masked key display
+- Default from address auto-set from admin email
+- No-JS support via controller POST fallback
+- 17 audit fixes (a11y, error handling, UX)
 
-**Files:** `lib/berrypod_web/live/admin/email_settings.ex`, shared components for guided flow
+**Session 2 (uncommitted):**
+- Dropped 4 transactional-only providers (Resend, Postmark, Mailgun, MailPace) — all are transactional-only and not useful for a shop that needs newsletters. 6 adapters remain: Brevo, SendGrid, Mailjet, MailerSend + SMTP, Postal
+- Removed "Popular providers" heading and description (unnecessary with fewer providers)
+- Fixed no-JS provider switching: noscript submit button always visible with dynamic text
+- Fixed no-JS radio highlight: replaced server-applied `card-radio-card-selected` class with pure CSS `:has(:checked)` selector
+- Cleaned up key validation (removed 4 provider clauses, removed 2 known prefixes)
+- Updated all related tests (adapters, key validation, email settings LiveView, controller, mailer)
+
+**Files:** `lib/berrypod_web/live/admin/email_settings.ex`, `lib/berrypod/mailer/adapters.ex`, `lib/berrypod/key_validation.ex`, `assets/css/admin/components.css`, `lib/berrypod_web/controllers/email_settings_controller.ex`
 
 ### E. Contextual prompts for skipped steps
 
@@ -148,7 +159,7 @@ Increase input field border contrast to meet WCAG AA (3:1 minimum for UI compone
 | A | Simplify initial setup to account creation only | 1.5h | planned |
 | B | Guided setup flow with progress bar | 4h | planned |
 | C | Forgiving API key validation | 1.5h | planned |
-| D | Email provider setup UX rework | 2h | planned |
+| D | Email provider setup UX rework | 2h | done |
 | E | Contextual prompts for skipped steps | 2h | done |
 | F | Dashboard checklist and messaging rework | 2h | planned |
 | G | Coming soon page fixes (logo + admin link) | 30m | planned |