Add chapter count badges, cover zoom, latest chapter button, and dismiss continue-reading cards

- Home page book cards show downloaded chapter count (amber badge) via new CountChapters method
- Book page cover image is clickable to zoom into a fullscreen overlay; click anywhere to dismiss
- Book page shows a 'Latest Chapter' button for unstarted books linking to the last downloaded chapter
- Continue-reading cards on home page have a dismiss (×) button that removes progress from localStorage and slides the card out to the left

scraper/internal/writer/writer.go

@@ -206,6 +206,25 @@ func (w *Writer) ListChapters(slug string) ([]ChapterInfo, error) {
 	return chapters, nil
 }
 
+// CountChapters returns the number of chapter markdown files on disk for slug.
+// It is cheaper than ListChapters because it does not read file contents.
+func (w *Writer) CountChapters(slug string) int {
+	bookDir := w.bookDir(slug)
+	volDirs, err := filepath.Glob(filepath.Join(bookDir, "vol-*"))
+	if err != nil {
+		return 0
+	}
+	count := 0
+	for _, vd := range volDirs {
+		rangeDirs, _ := filepath.Glob(filepath.Join(vd, "*-*"))
+		for _, rd := range rangeDirs {
+			files, _ := filepath.Glob(filepath.Join(rd, "chapter-*.md"))
+			count += len(files)
+		}
+	}
+	return count
+}
+
 // chapterTitle reads the first non-empty line of a markdown file and strips
 // the leading "# " heading marker. Falls back to "Chapter N".
 func chapterTitle(path string, n int) (title, date string) {