Completed studies: finishing a study keeps access after leaving; certificates always shown; no reminders for finished studies; v2.11.0

- Completion is stored on the user (completedStudySlugs, sticky) when every lesson is done; backfilled at startup
- Lessons, notes, progress, quiz, community and certificate stay open for completed studies after leaving the enrollment
- Dashboard/My Account list completed studies with Review Lessons and certificate access
- Study page shows a completed banner
- Lesson-release reminders, weekly digest and re-engagement emails skip finished studies
- Fix: streak fields (current/longest/last studied) and firstCompletionAt were dropped on every restart when users were loaded
- Tests for completion/access/re-engagement

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/App.css

@@ -4541,6 +4541,19 @@
 .account-cert-study .study-cert-heading { display: none; }
 .account-cert-study .study-cert-canvas { width: 100%; height: auto; max-width: 100%; }
 
+.account-chip--inline { margin-left: 0.5rem; vertical-align: middle; }
+
+.study-completed-banner {
+  max-width: 560px;
+  margin: 0 0 1.25rem;
+  padding: 0.75rem 1rem;
+  border: 1px solid rgba(111, 207, 151, 0.4);
+  border-radius: 12px;
+  background: rgba(111, 207, 151, 0.08);
+  color: #b9e2c9;
+  font-size: 0.95rem;
+}
+
 /* ── Study hub (guest landing) ── */
 .study-hub-count {
   margin: 1.25rem 0 0;