🤖 fix: require Node.js v20+ for Storybook (#470)

## Problem

Storybook was failing on this dev machine with:
```
SyntaxError: Identifier '__dirname' has already been declared
```

This occurred because Node.js v18.19.1 had a conflict with
esbuild-register when loading Vite code in Storybook's config.
esbuild-register was trying to declare `const __dirname` but it was
already declared in the environment.

## Solution

**Machine fix:** Upgraded Node.js from v18.19.1 to v20.19.4 using the
`n` version manager.

**Code fix:** Added Node.js version check to Makefile to prevent this
issue for others:
- Requires Node.js v20+ for all Storybook-related targets
- Provides clear error message with upgrade instructions
- Added `--no-open` flag to storybook dev to prevent xdg-open errors on
headless machines

## Verification

Storybook now starts successfully:
```
Storybook 8.6.14 for react-vite started
Local: http://localhost:6006/
```

Version check works correctly:
```bash
$ PATH="/old-node:$PATH" make storybook
Error: Node.js v20 or higher is required
Current version: v18
```

_Generated with `cmux`_

Author: ammar-agent

Makefile

@@ -46,6 +46,28 @@ include fmt.mk
 # Build tools
 TSGO := bun run node_modules/@typescript/native-preview/bin/tsgo.js
 
+# Node.js version check
+REQUIRED_NODE_VERSION := 20
+NODE_VERSION := $(shell node --version | sed 's/v\([0-9]*\).*/\1/')
+
+define check_node_version
+	@if [ "$(NODE_VERSION)" -lt "$(REQUIRED_NODE_VERSION)" ]; then \
+		echo "Error: Node.js v$(REQUIRED_NODE_VERSION) or higher is required"; \
+		echo "Current version: v$(NODE_VERSION)"; \
+		echo ""; \
+		echo "To upgrade Node.js:"; \
+		echo "  1. Install 'n' version manager: curl -L https://raw.githubusercontent.com/tj/n/master/bin/n | sudo bash -s -- lts"; \
+		echo "  2. Or use 'n' if already installed: sudo n $(REQUIRED_NODE_VERSION)"; \
+		echo ""; \
+		exit 1; \
+	fi
+endef
+
+# Detect if browser opener is available that Storybook can use
+# Storybook uses 'open' package which tries xdg-open on Linux, open on macOS, start on Windows
+HAS_BROWSER_OPENER := $(shell command -v xdg-open >/dev/null 2>&1 && echo "yes" || echo "no")
+STORYBOOK_OPEN_FLAG := $(if $(filter yes,$(HAS_BROWSER_OPENER)),,--no-open)
+
 TS_SOURCES := $(shell find src -type f \( -name '*.ts' -o -name '*.tsx' \))
 
 # Default target
@@ -244,15 +266,19 @@ docs-watch: ## Watch and rebuild documentation
 
 ## Storybook
 storybook: node_modules/.installed ## Start Storybook development server
-	@bun x storybook dev -p 6006
+	$(check_node_version)
+	@bun x storybook dev -p 6006 $(STORYBOOK_OPEN_FLAG)
 
 storybook-build: node_modules/.installed src/version.ts ## Build static Storybook
+	$(check_node_version)
 	@bun x storybook build
 
 test-storybook: node_modules/.installed ## Run Storybook interaction tests (requires Storybook to be running or built)
+	$(check_node_version)
 	@bun x test-storybook
 
 chromatic: node_modules/.installed ## Run Chromatic for visual regression testing
+	$(check_node_version)
 	@bun x chromatic --exit-zero-on-changes
 
 ## Benchmarks