valw: overview backdrop and window picker

modules/home/niri/desktop-niri-execs.ulu.nix

@@ -7,6 +7,8 @@
         "prefer-no-csd" = true;
         spawn-at-startup = [
           { command = [ "noctalia" ]; }
+          # valw's paint shader behind the overview (see the layer rule).
+          { command = [ "valw" "backdrop" ]; }
         ];
       };
     };

modules/home/niri/desktop-niri-general.ulu.nix

@@ -79,6 +79,12 @@
             ];
             place-within-backdrop = true;
           }
+          {
+            matches = [
+              { namespace = "^valw-backdrop$"; }
+            ];
+            place-within-backdrop = true;
+          }
         ];
       };
     };