add setup script

setup.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+set -e
+
+# Install mkcert if needed
+if ! command -v mkcert &>/dev/null; then
+    echo "Installing mkcert..."
+    brew install mkcert
+fi
+
+# Install the local CA into the system/browser trust stores
+mkcert -install
+
+# Generate certs for localhost
+mkdir -p certs
+mkcert -cert-file certs/localhost.pem -key-file certs/localhost-key.pem localhost 127.0.0.1
+
+echo "Setup complete. Run deno/dev.sh and ddln_cli/dev.sh to start."