One companion study guide per series; v2.9.8

- Second series gets its own guide (Resources page card, download page, admin fields)
- When a series ends (scheduled or manual) its guide moves onto its Finished Book, which shows a guide button; the main series' guide is cleared unless the launch supplies a new one
- Lane B launch schedule can supply the incoming series' guide
- Download lookup covers second-series and finished-book guides; guide URLs are sanitized on save and on use

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

server/routes/downloads.js

@@ -145,6 +145,18 @@ export function register(app) {
         const customResources = Array.isArray(siteContent.customLinks)
           ? siteContent.customLinks.filter(link => link?.placement === 'resources')
           : []
+        // Companion study guides: the second series' guide, and guides kept on finished books
+        if (id === 'guide:second') {
+          const guide = siteContent.secondSeries?.guide
+          const guideUrl = sanitizeUrl(guide?.downloadUrl)
+          return guideUrl ? { label: guide.title, url: guideUrl } : null
+        }
+        if (id.startsWith('guide:book:')) {
+          const bookId = id.slice('guide:book:'.length)
+          const book = Array.isArray(siteContent.finishedBooks) ? siteContent.finishedBooks.find(item => item?.id === bookId) : null
+          const bookGuideUrl = sanitizeUrl(book?.guide?.downloadUrl)
+          return bookGuideUrl ? { label: book.guide.title, url: bookGuideUrl } : null
+        }
         if (id.startsWith('custom:')) {
           const customId = id.slice('custom:'.length)
           const match = customResources.find(link => link.id === customId)