Make the timeline the default view, paginated and configurable

The day-grouped timeline is now the site root (/, /page/N/); the flat
square grid moves to /grid/ (/grid/page/N/). Both are linked from the
profile header, separated by a divider.

Timeline pages pack whole day-groups until they reach the page size,
never splitting a day across a page boundary, so appended pages never
repeat a date heading — the infinite-scroll handler relies on this.

Page size is configurable via -page-size (default 60), threaded through
the Generator and exposed as services.photogallery.pageSize in the
NixOS module.

main.go

@@ -19,6 +19,7 @@ func main() {
 	fontsURL   := flag.String("fonts-url", "https://fonts.googleapis.com/css2?family=Crimson+Pro:ital,wght@0,400;0,600;0,700;1,400&family=Manjari:wght@100;400;700&display=swap", "stylesheet URL loading the two web fonts")
 	serifFamily := flag.String("serif-family", "Crimson Pro", "CSS font-family for the primary serif text")
 	mlFamily    := flag.String("ml-family", "Manjari", "CSS font-family for the alt-script (Malayalam) bio line")
+	pageSize    := flag.Int("page-size", 60, "photos per page on the timeline and grid views (timeline packs whole days, so pages vary slightly around this)")
 	flag.Parse()
 
 	for _, d := range []string{*contentDir, *outputDir} {
@@ -27,7 +28,7 @@ func main() {
 		}
 	}
 
-	g := NewGenerator(*contentDir, *outputDir, *baseURL, *author, *handle, *bio, *bioAlt, *fontsURL, *serifFamily, *mlFamily)
+	g := NewGenerator(*contentDir, *outputDir, *baseURL, *author, *handle, *bio, *bioAlt, *fontsURL, *serifFamily, *mlFamily, *pageSize)
 
 	if err := g.Build(); err != nil {
 		log.Printf("initial build: %v", err)