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>

PROGRESS.md

@@ -24,7 +24,7 @@ Tier 1 MVP complete. Tier 2 production readiness complete (except Litestream and
 - Abandoned cart recovery (GDPR-compliant, single email)
 - Favicon generation from source image (PNG variants, SVG dark mode, webmanifest)
 - Complete SEO (OG/Twitter cards, JSON-LD, sitemap, canonical URLs, meta descriptions)
-- Email settings admin with 10 adapter options and test email
+- Email settings admin with 6 adapter options, no-JS support, and test email
 - No-JS support across all key flows
 - Fully Tailwind-free CSS (12 KB gzipped shop+theme, 95 KB gzipped admin total)
 - CI pipeline (compile warnings, format, credo, dialyzer, tests)
@@ -60,7 +60,7 @@ Based on usability testing (March 2026). Reworks the entire setup flow into a si
 | A | Simplify initial setup to account creation only (email, password, shop name) | 1.5h | planned |
 | B | Guided setup flow with progress bar (multi-step, skippable, explains "why") | 4h | planned |
 | C | Forgiving API key validation (strip whitespace, format checks, helpful errors) | 1.5h | done |
-| D | Email provider setup UX rework (recommended pick, grouping, guided flow, test email) | 2h | planned |
+| D | Email provider setup UX rework (6 adapters, no-JS, `:has(:checked)` CSS, test email) | 2h | done |
 | E | Contextual prompts for skipped steps (products, checkout, order detail) | 2h | done |
 | F | Dashboard checklist and messaging rework | 2h | planned |
 | G | Coming soon page fixes (logo layout, admin login link) | 30m | done |