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.

module.nix

@@ -76,6 +76,12 @@ in {
       description = "CSS font-family for the alt-script (Malayalam) bio line. Must match a family loaded by fontsURL.";
     };
 
+    pageSize = lib.mkOption {
+      type        = lib.types.ints.positive;
+      default     = 60;
+      description = "Photos per page on the timeline and grid views. The timeline packs whole days, so pages vary slightly around this.";
+    };
+
     nginx = {
       enable = lib.mkEnableOption "nginx virtual host for photogallery";
 
@@ -141,6 +147,7 @@ in {
             "--fonts-url"     cfg.fontsURL
             "--serif-family"  cfg.serifFamily
             "--ml-family"     cfg.mlFamily
+            "--page-size"     (toString cfg.pageSize)
           ];
 
           Restart    = "on-failure";