build(nodejs): enforce Node.js >=20 in engines and .nvmrc

reason: https://github.com/tangly1024/NotionNext/pull/3374#issuecomment-2879323753

BREAKING CHANGE:
- Node.js 20 is now the minimum required version
- Updated .nvmrc and package.json "engines"
- Please upgrade local environment or CI setup

.npmrc

@@ -0,0 +1 @@
+engine-strict=true

Dockerfile

@@ -1,7 +1,7 @@
 ARG NOTION_PAGE_ID
 ARG NEXT_PUBLIC_THEME
 
-FROM node:18-alpine3.18 AS base
+FROM node:20-alpine AS base
 
 # 1. Install dependencies only when needed
 FROM base AS deps

package.json

@@ -3,6 +3,9 @@
   "version": "4.8.4",
   "homepage": "https://github.com/tangly1024/NotionNext.git",
   "license": "MIT",
+  "engines": {
+    "node": ">=20"
+  },
   "repository": {
     "type": "git",
     "url": "https://github.com/tangly1024/NotionNext.git"