Add study account engagement features; v1.2.4

- Pause reminders: remindersPausedUntil field + scheduler skip + account UI with 7/14/30-day presets
- Email digest mode: reminderFrequency field (immediate/weekly) + weekly digest email + scheduler + account UI toggle
- Admin user list improvements: streak, reengagement, frequency, pause, per-study lesson progress
- Admin manual re-engagement: POST endpoint + tier selector button in admin panel
- Study bookmarks: bookmarkedSectionIds in progress file + POST/DELETE endpoints + toggle button in lesson UI
- Study completion certificate email: auto-sent on first certificate issuance
- Content fields for certificate and weekly digest email templates in admin

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

server/routes/study-certificate.js

@@ -4,6 +4,8 @@ import { requireAdminAuth } from '../auth.js'
 import { requireStudyAuth, normalizeStudySlug, isStudyUserEnrolled, getStudyTitleBySlug } from '../study-helpers.js'
 import { state } from '../state.js'
 import { loadUserProgress, queueStudyCertificatesWrite } from '../data.js'
+import { sendStudyCertificateEmail, getCanonicalBaseUrl } from '../email.js'
+import { buildAbsoluteUrl } from '../helpers.js'
 
 const publicCertRateLimiter = rateLimit({
   windowMs: 15 * 60 * 1000,
@@ -95,6 +97,9 @@ export function register(app) {
       }
       state.studyCertificates.push(cert)
       queueStudyCertificatesWrite()
+      const certUrl = buildAbsoluteUrl(getCanonicalBaseUrl(), `/study/certificate/${cert.token}`)
+      sendStudyCertificateEmail(user.username, user.displayName || user.username, cert.studyTitle, certUrl)
+        .catch(err => console.error('[study-certificate] email exception:', err))
     }
 
     res.json({ ok: true, token: cert.token, issuedAt: cert.issuedAt })