Upgrade project to Node 24

Node 22 leaves active LTS at the end of October. Move the Docker base
image and the documented local requirement to Node 24.

Pull @types/node from ^25 down to ^24 so the type definitions no longer
describe a newer runtime than the one in use. Types ahead of the runtime
let TypeScript accept APIs that are not there at execution time; the
opposite direction is harmless.

Add an engines field to record the supported version. No .nvmrc, since
the project is not used with a Node version manager.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

package.json

@@ -3,6 +3,9 @@
   "version": "1.0.0",
   "description": "Spreadsheet-style circuit list editor for electrical distribution planning",
   "main": "dist/server/index.js",
+  "engines": {
+    "node": "24.x"
+  },
   "scripts": {
     "dev": "npm run dev:api",
     "dev:api": "tsx watch src/server/index.ts",
@@ -40,7 +43,7 @@
   "devDependencies": {
     "@types/better-sqlite3": "^7.6.13",
     "@types/express": "^5.0.6",
-    "@types/node": "^25.6.0",
+    "@types/node": "^24.10.1",
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
     "drizzle-kit": "^0.31.10",