webapp-shell: use Node 22 instead of end-of-life Node 20

nodejs_20 in webapp-shell sat in packages, ahead of the inherited rainix
build inputs on PATH, so script/vercel-build.sh and the ui-components
screenshot harness ignored the patched Node 22 from rainix. packages/raindex
already declares engines.node >=22.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: Siddharth2207

flake.nix

@@ -323,7 +323,7 @@
           inherit (rainix.devShells.${system}.default) shellHook buildInputs nativeBuildInputs;
         };
         devShells.webapp-shell = pkgs.mkShell {
-          packages = with pkgs; [ nodejs_20 ];
+          packages = with pkgs; [ nodejs_22 ];
           inherit (rainix.devShells.${system}.default) buildInputs nativeBuildInputs;
         };