Add newer/older navigation to the photo detail page

Replace the standalone Back link with a three-cell nav row so each
photo page links to its neighbours in the timeline. Prefetch the
neighbouring pages and bind the arrow keys for keyboard navigation.

generator.go

@@ -108,8 +108,15 @@ func (g *Generator) Build() error {
 	if err := g.renderIndexPages(photos); err != nil {
 		return err
 	}
-	for _, p := range photos {
-		if err := g.renderPhotoPage(p); err != nil {
+	for i := range photos {
+		var newer, older *Photo
+		if i > 0 {
+			newer = &photos[i-1]
+		}
+		if i+1 < len(photos) {
+			older = &photos[i+1]
+		}
+		if err := g.renderPhotoPage(photos[i], newer, older); err != nil {
 			return err
 		}
 	}
@@ -358,13 +365,15 @@ func (g *Generator) renderIndexPages(photos []Photo) error {
 }
 
 // renderPhotoPage writes public/photo/<slug>/index.html.
-func (g *Generator) renderPhotoPage(p Photo) error {
+func (g *Generator) renderPhotoPage(p Photo, newer, older *Photo) error {
 	dir := filepath.Join(g.outputDir, "photo", p.Slug)
 	if err := os.MkdirAll(dir, 0755); err != nil {
 		return err
 	}
 	return g.render("photo.html", filepath.Join(dir, "index.html"), map[string]any{
 		"Photo":       p,
+		"Newer":       newer,
+		"Older":       older,
 		"BaseURL":     g.baseURL,
 		"Year":        time.Now().Year(),
 		"Author":      g.author,