feat: add template helper functions for book detail page

Add utility functions for rendering book metadata:

- renderStars(): Convert rating (1-10 scale) to star display
- formatFileSize(): Convert bytes to human-readable format (KB, MB, GB)
- getExternalURL(): Generate URLs for external book services
  with smart fallback: ID → ISBN → Title+Author search
  Supports Goodreads, Open Library, Google Books, Amazon

These helpers make book detail template cleaner and follow DRY principle.

templates/utils.go

@@ -1,6 +1,8 @@
 package templates
 
 import (
+	"fmt"
+
 	"github.com/google/uuid"
 	"github.com/jackc/pgx/v5/pgtype"
 )
@@ -32,3 +34,91 @@ func uuidToString(id pgtype.UUID) string {
 	}
 	return u.String()
 }
+
+// renderStars converts rating (1-10 scale) to star display
+// Rating scale: 1-10 where odd numbers = half stars (1=0.5★, 3=1.5★, etc.)
+func renderStars(rating int32) string {
+	stars := ""
+	fullStars := rating / 2
+	hasHalf := rating%2 != 0
+
+	for i := int32(0); i < fullStars; i++ {
+		stars += "★"
+	}
+	if hasHalf {
+		stars += "½"
+	}
+
+	return stars
+}
+
+// formatFileSize converts bytes to human-readable format
+func formatFileSize(bytes int64) string {
+	const (
+		KB = 1024
+		MB = KB * 1024
+		GB = MB * 1024
+	)
+
+	switch {
+	case bytes >= GB:
+		return fmt.Sprintf("%.2f GB", float64(bytes)/float64(GB))
+	case bytes >= MB:
+		return fmt.Sprintf("%.2f MB", float64(bytes)/float64(MB))
+	case bytes >= KB:
+		return fmt.Sprintf("%.2f KB", float64(bytes)/float64(KB))
+	default:
+		return fmt.Sprintf("%d B", bytes)
+	}
+}
+
+// getExternalURL generates URL for external book services
+// Priority: ID > ISBN > Title+Author search
+func getExternalURL(service string, id string, isbn pgtype.Text, title string, author pgtype.Text) string {
+	baseURL := ""
+	searchTerm := ""
+
+	// Determine search term: ID > ISBN > Title+Author
+	if id != "" {
+		searchTerm = id
+	} else if isbn.Valid && isbn.String != "" {
+		searchTerm = isbn.String
+	} else {
+		// Build title+author search query
+		if author.Valid && author.String != "" {
+			searchTerm = fmt.Sprintf("%s %s", title, author.String)
+		} else {
+			searchTerm = title
+		}
+	}
+
+	// Build URL based on service
+	switch service {
+	case "goodreads":
+		if id != "" {
+			baseURL = "https://www.goodreads.com/book/show/"
+		} else {
+			baseURL = "https://www.goodreads.com/search?q="
+		}
+	case "openlibrary":
+		if id != "" {
+			baseURL = "https://openlibrary.org/books/"
+		} else {
+			baseURL = "https://openlibrary.org/search?q="
+		}
+	case "googlebooks":
+		if id != "" {
+			baseURL = "https://books.google.com/books?id="
+		} else {
+			baseURL = "https://www.google.com/search?tbm=bks&q="
+		}
+	case "amazon":
+		// Amazon doesn't have direct book IDs, always search
+		baseURL = "https://www.amazon.com/s?k="
+		if isbn.Valid && isbn.String != "" {
+			searchTerm = isbn.String
+		}
+	}
+
+	return baseURL + searchTerm
+}