Paginate the gallery grid with 60 photos per page

Emit /page/N/ pages alongside index.html and render a pager at the
bottom of each page. Ships as a plain link that navigates without JS,
and an IntersectionObserver enhancement that fetches the next page and
appends tiles for an infinite-scroll feel.

generator.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"path/filepath"
 	"sort"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -23,6 +24,7 @@ import (
 const (
 	thumbSize     = 600
 	mediumMaxSize = 1600
+	pageSize      = 60
 )
 
 // PhotoMeta is the structure of a .toml sidecar file.
@@ -103,7 +105,7 @@ func (g *Generator) Build() error {
 		}
 	}
 
-	if err := g.renderIndex(photos); err != nil {
+	if err := g.renderIndexPages(photos); err != nil {
 		return err
 	}
 	for _, p := range photos {
@@ -290,21 +292,69 @@ func generateMedium(src, dst string, maxDim int) error {
 	return jpeg.Encode(out, resized, &jpeg.Options{Quality: 88})
 }
 
-// renderIndex writes public/index.html.
-func (g *Generator) renderIndex(photos []Photo) error {
-	return g.render("index.html", filepath.Join(g.outputDir, "index.html"), map[string]any{
-		"Photos":      photos,
-		"BaseURL":     g.baseURL,
-		"Year":        time.Now().Year(),
-		"Author":      g.author,
-		"Handle":      g.handle,
-		"Bio":         g.bio,
-		"BioAlt":      g.bioAlt,
-		"Initial":     g.initial(),
-		"FontsURL":    g.fontsURL,
-		"SerifFamily": g.serifFamily,
-		"MlFamily":    g.mlFamily,
-	})
+// renderIndexPages writes public/index.html and public/page/N/index.html
+// files, chunking photos into pageSize slices for progressive pagination.
+func (g *Generator) renderIndexPages(photos []Photo) error {
+	total := len(photos)
+	totalPages := (total + pageSize - 1) / pageSize
+	if totalPages == 0 {
+		totalPages = 1
+	}
+
+	for i := 0; i < totalPages; i++ {
+		start := i * pageSize
+		end := start + pageSize
+		if end > total {
+			end = total
+		}
+		slice := photos[start:end]
+
+		dst := filepath.Join(g.outputDir, "index.html")
+		if i > 0 {
+			dir := filepath.Join(g.outputDir, "page", strconv.Itoa(i+1))
+			if err := os.MkdirAll(dir, 0755); err != nil {
+				return err
+			}
+			dst = filepath.Join(dir, "index.html")
+		}
+
+		prev := ""
+		next := ""
+		switch i {
+		case 0:
+			// no prev
+		case 1:
+			prev = "/"
+		default:
+			prev = "/page/" + strconv.Itoa(i) + "/"
+		}
+		if i+1 < totalPages {
+			next = "/page/" + strconv.Itoa(i+2) + "/"
+		}
+
+		data := map[string]any{
+			"Photos":      slice,
+			"Total":       total,
+			"Page":        i + 1,
+			"TotalPages":  totalPages,
+			"PrevURL":     prev,
+			"NextURL":     next,
+			"BaseURL":     g.baseURL,
+			"Year":        time.Now().Year(),
+			"Author":      g.author,
+			"Handle":      g.handle,
+			"Bio":         g.bio,
+			"BioAlt":      g.bioAlt,
+			"Initial":     g.initial(),
+			"FontsURL":    g.fontsURL,
+			"SerifFamily": g.serifFamily,
+			"MlFamily":    g.mlFamily,
+		}
+		if err := g.render("index.html", dst, data); err != nil {
+			return err
+		}
+	}
+	return nil
 }
 
 // renderPhotoPage writes public/photo/<slug>/index.html.