feat(dev): add live-reload dev server with Juice CSS inlining

- tools/server/: Node.js dev server (Express + WebSocket + fs.watch)
  Watches themes/ for .tmpl changes, rebuilds via Go preview command,
  applies Juice CSS inlining for email client compatibility, pushes
  live reload to browser via WebSocket.
- tools/cli/dev.go: 'go run . dev' subcommand — starts Node.js server
  with automatic npm install, initial preview generation, and
  Ctrl+C graceful shutdown.
- tools/tools.go: register dev command in CLI app.
- .gitignore: add node_modules exclusion.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/cli/dev.go

@@ -0,0 +1,102 @@
+package cli
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"os/signal"
+	"path/filepath"
+	"syscall"
+
+	"github.com/urfave/cli/v2"
+)
+
+// DevCommand returns the "dev" subcommand.
+func DevCommand() *cli.Command {
+	return &cli.Command{
+		Name:  "dev",
+		Usage: "Start a live-reload dev server with CSS inlining (requires Node.js)",
+		UsageText: `go run . dev [--port <port>]`,
+		Flags: []cli.Flag{
+			&cli.IntFlag{
+				Name:  "port",
+				Value: 3456,
+				Usage: "Port for the dev server",
+			},
+		},
+		Action: runDev,
+	}
+}
+
+func runDev(c *cli.Context) error {
+	port := c.Int("port")
+
+	// Find the server directory relative to the tools/ dir
+	exe, _ := os.Executable()
+	toolsDir := filepath.Dir(exe)
+	// When run with `go run`, resolve relative to CWD
+	if _, err := os.Stat(filepath.Join(toolsDir, "server")); os.IsNotExist(err) {
+		cwd, _ := os.Getwd()
+		toolsDir = cwd
+	}
+	serverDir := filepath.Join(toolsDir, "server")
+
+	// Verify Node.js is available
+	if _, err := exec.LookPath("node"); err != nil {
+		return fmt.Errorf("Node.js is required but not found in PATH")
+	}
+
+	// Verify node_modules are installed
+	if _, err := os.Stat(filepath.Join(serverDir, "node_modules")); os.IsNotExist(err) {
+		fmt.Println("Installing Node.js dependencies...")
+		cmd := exec.Command("npm", "install")
+		cmd.Dir = serverDir
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+		if err := cmd.Run(); err != nil {
+			return fmt.Errorf("npm install failed: %w", err)
+		}
+	}
+
+	// Ensure rendered.js exists before starting
+	initCmd := exec.Command("go", "run", ".", "preview", "all")
+	initCmd.Dir = toolsDir
+	initCmd.Stdout = os.Stdout
+	initCmd.Stderr = os.Stderr
+	if err := initCmd.Run(); err != nil {
+		fmt.Println("Warning: initial preview generation failed, starting anyway...")
+	}
+
+	// Start the Node.js dev server
+	serverCmd := exec.Command("node", "server.mjs")
+	serverCmd.Dir = serverDir
+	serverCmd.Env = append(os.Environ(), fmt.Sprintf("PORT=%d", port))
+	serverCmd.Stdout = os.Stdout
+	serverCmd.Stderr = os.Stderr
+
+	if err := serverCmd.Start(); err != nil {
+		return fmt.Errorf("failed to start dev server: %w", err)
+	}
+
+	fmt.Printf("\nDev server running at http://localhost:%d\n", port)
+	fmt.Print("Press Ctrl+C to stop.\n\n")
+
+	// Handle graceful shutdown
+	sigCh := make(chan os.Signal, 1)
+	signal.Notify(sigCh, syscall.SIGINT, syscall.SIGTERM)
+
+	go func() {
+		<-sigCh
+		fmt.Println("\nShutting down...")
+		if err := serverCmd.Process.Signal(os.Interrupt); err != nil {
+			serverCmd.Process.Kill()
+		}
+		os.Exit(0)
+	}()
+
+	if err := serverCmd.Wait(); err != nil {
+		return fmt.Errorf("dev server exited: %w", err)
+	}
+
+	return nil
+}