Add single-day pages linked from timeline date headings

Each timeline date heading now links to /day/<YYYY-MM-DD>/, a page
holding just that day's photos with a back link to the timeline.
renderDayPages writes one per groupByDay run; the ISO label doubles
as the URL slug.

generator.go

@@ -106,7 +106,7 @@ func (g *Generator) Build() error {
 		return err
 	}
 
-	for _, d := range []string{"images", "thumbs", "medium", "photo", "grid", "static"} {
+	for _, d := range []string{"images", "thumbs", "medium", "photo", "grid", "day", "static"} {
 		if err := os.MkdirAll(filepath.Join(g.outputDir, d), 0755); err != nil {
 			return err
 		}
@@ -124,6 +124,9 @@ func (g *Generator) Build() error {
 	if err := g.renderGridPages(photos); err != nil {
 		return err
 	}
+	if err := g.renderDayPages(photos); err != nil {
+		return err
+	}
 	for i := range photos {
 		var newer, older *Photo
 		if i > 0 {
@@ -508,6 +511,37 @@ func (g *Generator) renderTimelinePages(photos []Photo) error {
 	return nil
 }
 
+// renderDayPages writes one page per calendar day under public/day/<ISO>/,
+// holding just that day's photos. Timeline date headings link here.
+func (g *Generator) renderDayPages(photos []Photo) error {
+	for _, grp := range groupByDay(photos) {
+		dir := filepath.Join(g.outputDir, "day", grp.day)
+		if err := os.MkdirAll(dir, 0755); err != nil {
+			return err
+		}
+		err := g.render("day.html", filepath.Join(dir, "index.html"), map[string]any{
+			"Date":        grp.Label,
+			"Count":       grp.Count,
+			"Photos":      grp.Photos,
+			"Total":       len(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,
+		})
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 func (g *Generator) render(tmpl, dst string, data any) error {
 	f, err := os.Create(dst)
 	if err != nil {