Add one-command setup via nix run

- Add flake app so setup can be run with: nix run .
- Update README with comprehensive setup guide for:
  - First computer (initial setup)
  - Additional computers (joining)
  - Mobile device pairing
  - NixOS module usage

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.nix

@@ -76,6 +76,54 @@
       #   };
       # };
 
+      # One-command setup: nix run .
+      apps = forAllSystems (system:
+        let
+          pkgs = nixpkgsFor.${system};
+          setupScript = pkgs.writeShellApplication {
+            name = "unsbs-setup";
+            runtimeInputs = with pkgs; [
+              git
+              jq
+              age
+              sops
+              syncthing
+              qrencode
+              zbar
+              ffmpeg
+            ];
+            text = ''
+              # Find the setup script
+              SCRIPT_DIR="''${UNSBS_DIR:-}"
+              if [[ -z "$SCRIPT_DIR" ]]; then
+                # Try current directory first
+                if [[ -f "./setup" ]]; then
+                  SCRIPT_DIR="."
+                # Try the flake source
+                elif [[ -f "${self}/setup" ]]; then
+                  SCRIPT_DIR="${self}"
+                else
+                  echo "Error: Cannot find setup script"
+                  echo "Run from the repo directory, or clone it first:"
+                  echo "  git clone https://github.com/YOUR_USERNAME/ultimate-notetaking-sync-backup-system.git"
+                  exit 1
+                fi
+              fi
+              exec "$SCRIPT_DIR/setup" "$@"
+            '';
+          };
+        in {
+          default = {
+            type = "app";
+            program = "${setupScript}/bin/unsbs-setup";
+          };
+          setup = {
+            type = "app";
+            program = "${setupScript}/bin/unsbs-setup";
+          };
+        }
+      );
+
       # Development shell with all tools
       devShells = forAllSystems (system:
         let