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>

src/content.ts

@@ -259,6 +259,15 @@ export interface SiteContent {
   studyReengagementT3Subject: string
   studyReengagementT3Body: string
   studyReengagementT3Cta: string
+  // ── Study Completion Certificate Email ──
+  studyCertificateEmailSubject: string
+  studyCertificateEmailBody: string
+  studyCertificateEmailCtaLabel: string
+  studyCertificateEmailSignoff: string
+  // ── Study Weekly Lesson Digest Email ──
+  studyWeeklyDigestEmailSubject: string
+  studyWeeklyDigestEmailBody: string
+  studyWeeklyDigestEmailSignoff: string
   // ── Email Change Confirmation ──
   emailChangeSubject: string
   emailChangeBody: string
@@ -472,6 +481,13 @@ export const DEFAULTS: SiteContent = {
   studyReengagementT3Subject: 'Your progress is still here',
   studyReengagementT3Body: "Your study progress is still intact and waiting. There's no deadline — come back whenever you're ready.",
   studyReengagementT3Cta: 'Open Your Study',
+  studyCertificateEmailSubject: 'Your certificate is ready',
+  studyCertificateEmailBody: 'Congratulations — you have completed every lesson in this study. Your certificate is ready to view and share.',
+  studyCertificateEmailCtaLabel: 'View Certificate',
+  studyCertificateEmailSignoff: 'Grace and peace,\nVerse by Verse with Nate',
+  studyWeeklyDigestEmailSubject: 'Your weekly study digest',
+  studyWeeklyDigestEmailBody: 'Here are the lessons released this week in your study track. Open any to continue where you left off.',
+  studyWeeklyDigestEmailSignoff: 'Grace and peace,\nVerse by Verse with Nate',
   emailChangeSubject: 'Confirm your new email address',
   emailChangeBody: 'Click the link below to confirm your new account email. If you did not request this change, ignore this message.',
   emailChangeCtaLabel: 'Confirm Email Change',