Studies: add a Download Study Guide button to the study page, automatic from the linked series/book or chosen per study in admin; v2.35.0

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

server/helpers.js

@@ -344,6 +344,7 @@ function sanitizeStudies(value) {
       showOnHomepage: item.showOnHomepage === true,
       showNewTag: item.showNewTag === true,
       newTagLabel: typeof item.newTagLabel === 'string' ? item.newTagLabel.trim().slice(0, 24) : '',
+      guideSource: typeof item.guideSource === 'string' && /^(none|primary|second|book:[A-Za-z0-9_-]{1,80})$/.test(item.guideSource.trim()) ? item.guideSource.trim() : '',
       status: item.status === 'planned' ? 'planned' : (item.status === 'complete' ? 'complete' : 'active'),
       lane: item.lane === 'A' || item.lane === 'B' ? item.lane : '',
       startsAt: sanitizeIsoDate(item.startsAt),