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/server/package.json

@@ -0,0 +1,16 @@
+{
+  "name": "gitea-mail-templates-dev",
+  "version": "1.0.0",
+  "private": true,
+  "description": "Live-reload dev server with Juice CSS inlining for Gitea Mail Templates",
+  "type": "module",
+  "scripts": {
+    "start": "node server.mjs",
+    "inline": "node inliner.mjs"
+  },
+  "dependencies": {
+"express": "^5.1.0",
+    "express-ws": "^5.0.2",
+    "juice": "^11.0.0"
+  }
+}