add contextual prompts for skipped setup steps

Disable checkout when Stripe isn't connected (cart drawer, cart page,
and early guard in checkout controller to prevent orphaned orders).
Show amber warning on order detail when email isn't configured.
Fix pre-existing missing vertical spacing between page blocks.

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

PROGRESS.md

@@ -29,7 +29,7 @@ Tier 1 MVP complete. Tier 2 production readiness complete (except Litestream and
 - Fully Tailwind-free CSS (12 KB gzipped shop+theme, 95 KB gzipped admin total)
 - CI pipeline (compile warnings, format, credo, dialyzer, tests)
 - Deployed on Fly.io with observability (LiveDashboard, ErrorTracker, structured logging)
-- 1679+ tests passing, 99-100 PageSpeed mobile
+- 1716+ tests passing, 99-100 PageSpeed mobile
 
 ## Next up
 
@@ -59,9 +59,9 @@ 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 | 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 |
-| E | Contextual prompts for skipped steps (products, checkout, order detail) | 2h | planned |
+| 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 |
 | H | External links UX (new tabs, icons, aria labels) | 1h | done |