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.

.gitignore

@@ -1,5 +1,6 @@
 /content/
 /public/
+/.demo/
 # Anchored: unanchored these would also match the Kotlin package directory
 # android/app/src/main/java/ws/inflo/photogallery/ and any nested "result".
 /photogallery