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-account.js

@@ -218,6 +218,8 @@ export function register(app) {
         displayName: user.displayName ?? '',
         subscribeNewsletter: user.subscribeNewsletter !== false,
         studyRemindersEnabled: user.studyRemindersEnabled === true,
+        reminderFrequency: user.reminderFrequency ?? 'immediate',
+        remindersPausedUntil: user.remindersPausedUntil ?? null,
         avatarUrl: getStudyAvatarUrl(user),
       },
       stats: {
@@ -303,6 +305,20 @@ export function register(app) {
     const studyRemindersEnabled = req.body?.studyRemindersEnabled === true
     user.subscribeNewsletter = subscribeNewsletter
     user.studyRemindersEnabled = studyRemindersEnabled
+
+    const reminderFrequency = req.body?.reminderFrequency
+    if (reminderFrequency === 'weekly' || reminderFrequency === 'immediate') {
+      user.reminderFrequency = reminderFrequency
+    }
+
+    const rawPaused = req.body?.remindersPausedUntil
+    if (rawPaused === null || rawPaused === '') {
+      user.remindersPausedUntil = null
+    } else if (typeof rawPaused === 'string') {
+      const parsed = new Date(rawPaused)
+      user.remindersPausedUntil = !isNaN(parsed.getTime()) && parsed.getTime() > Date.now() ? parsed.toISOString() : null
+    }
+
     user.updatedAt = new Date().toISOString()
     queueStudyUsersWrite()
 
@@ -310,7 +326,13 @@ export function register(app) {
       syncContactToResend(user.displayName || user.username, user.username).catch(err => console.error('[study-account] resend sync error:', err))
     }
 
-    res.json({ ok: true, subscribeNewsletter: user.subscribeNewsletter, studyRemindersEnabled: user.studyRemindersEnabled === true })
+    res.json({
+      ok: true,
+      subscribeNewsletter: user.subscribeNewsletter,
+      studyRemindersEnabled: user.studyRemindersEnabled === true,
+      reminderFrequency: user.reminderFrequency ?? 'immediate',
+      remindersPausedUntil: user.remindersPausedUntil ?? null,
+    })
   })
 
   app.post('/api/study-account/request-email-change', studyAuthRateLimiter, requireStudyAuth, async (req, res) => {