nix: set up project-wide formatter

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I4806c58aa0a17f504c9312723ad770166a6a6964

flake.nix

@@ -21,5 +21,38 @@
     in {
       default = pkgs.callPackage ./nix/shell.nix {};
     });
+
+    formatter = forEachSystem (system: let
+      pkgs = nixpkgs.legacyPackages.${system};
+    in
+      pkgs.writeShellApplication {
+        name = "nix3-fmt-wrapper";
+
+        runtimeInputs = [
+          pkgs.alejandra
+          pkgs.fd
+          pkgs.prettier
+          pkgs.deno
+          pkgs.taplo
+          pkgs.sql-formatter
+        ];
+
+        text = ''
+          # Format Nix with Alejandra
+          fd "$@" -t f -e nix -x alejandra -q '{}'
+
+          # Format TOML with Taplo
+          fd "$@" -t f -e toml -x taplo fmt '{}'
+
+          # Format CSS with Prettier
+          fd "$@" -t f -e css -x prettier --write '{}'
+
+          # Format SQL with sql-format
+          fd "$@" -t f -e sql -x sql-formatter --fix '{}' -l postgresql
+
+          # Format Markdown with Deno
+          fd "$@" -t f -e md -x deno fmt -q '{}'
+        '';
+      });
   };
 }