feat(generator): add -default-view to choose timeline or grid as site root

The chosen view is published at /, the other under /timeline/ or
/grid/. Page paths, pager links and the cross-links between the views
are derived from the setting. The directory named after the default
view is removed on build, since it can only be a leftover from before
a switch.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

module.nix

@@ -82,6 +82,12 @@ in {
       description = "Photos per page on the timeline and grid views. The timeline packs whole days, so pages vary slightly around this.";
     };
 
+    defaultView = lib.mkOption {
+      type        = lib.types.enum [ "timeline" "grid" ];
+      default     = "timeline";
+      description = "View published at the site root. The other one is published under /timeline/ or /grid/.";
+    };
+
     nginx = {
       enable = lib.mkEnableOption "nginx virtual host for photogallery";
 
@@ -148,6 +154,7 @@ in {
             "--serif-family"  cfg.serifFamily
             "--ml-family"     cfg.mlFamily
             "--page-size"     (toString cfg.pageSize)
+            "--default-view"  cfg.defaultView
           ];
 
           Restart    = "on-failure";