test: make playwright stack self-contained on fresh hosts

apps/ui/scripts/playwright-stack.sh

@@ -17,6 +17,19 @@ POSTGRES_PASSWORD="${CRANK_E2E_POSTGRES_PASSWORD:-crank}"
 ADMIN_EMAIL="${CRANK_E2E_ADMIN_EMAIL:-owner@crank.local}"
 ADMIN_PASSWORD="${CRANK_E2E_ADMIN_PASSWORD:-change-me-admin-password}"
 
+if [[ -f "$HOME/.cargo/env" ]]; then
+  . "$HOME/.cargo/env"
+fi
+
+PYTHON_BIN="${PYTHON_BIN:-}"
+if [[ -z "$PYTHON_BIN" ]]; then
+  if command -v python3 >/dev/null 2>&1; then
+    PYTHON_BIN="python3"
+  else
+    PYTHON_BIN="python"
+  fi
+fi
+
 mkdir -p "$LOG_DIR"
 
 (
@@ -68,7 +81,7 @@ cleanup
 wait_for_port() {
   local host="$1"
   local port="$2"
-  until python - "$host" "$port" <<'PY'
+  until "$PYTHON_BIN" - "$host" "$port" <<'PY'
 import socket, sys
 sock = socket.socket()
 sock.settimeout(0.5)