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/data.js

@@ -1415,6 +1415,9 @@ export function sanitizeStudyUsers(value) {
       totpEnabledAt: typeof item?.totpEnabledAt === 'string' ? item.totpEnabledAt : null,
       totpRecoveryCodes: Array.isArray(item?.totpRecoveryCodes) ? item.totpRecoveryCodes.filter(h => typeof h === 'string') : [],
       totpSecretPending: typeof item?.totpSecretPending === 'string' ? item.totpSecretPending : undefined,
+      remindersPausedUntil: typeof item?.remindersPausedUntil === 'string' ? item.remindersPausedUntil : null,
+      reminderFrequency: item?.reminderFrequency === 'weekly' ? 'weekly' : 'immediate',
+      weeklyDigestSentAt: typeof item?.weeklyDigestSentAt === 'string' ? item.weeklyDigestSentAt : null,
     })
   }
   return out.slice(0, MAX_STUDY_USERS)
@@ -1459,8 +1462,12 @@ export function sanitizeStudyProgress(value) {
             ])
         )
         : {}
+      const bookmarkedSectionIds = Array.isArray(studyData?.bookmarkedSectionIds)
+        ? Array.from(new Set(studyData.bookmarkedSectionIds.filter(id => typeof id === 'string' && id.trim()).map(id => id.trim())))
+        : []
       progress.byStudy[studySlug.trim().toLowerCase()] = {
         completedSectionIds: Array.from(new Set(completedSectionIds)),
+        bookmarkedSectionIds,
         quizAnswers,
       }
     }