Unify Node.js version configuration

Use package.json engines field as the single source of truth for CI and
Babel, reducing the number of places where Node version is hardcoded.

Changes:
- Add engines.node to backend and frontend package.json
- Update CI workflows to use node-version-file instead of hardcoded version
- Convert .babelrc to babel.config.js that reads version from package.json
- Add engineComment to remind about serverless.yml sync

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: smessmer

.github/workflows/backend.yml

@@ -22,7 +22,7 @@ jobs:
       - name: Use Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: 20.x
+          node-version-file: 'backend/package.json'
           cache: npm
           cache-dependency-path: backend/package.json
       - name: Install Dependencies
@@ -54,7 +54,7 @@ jobs:
       - name: Use Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: 20.x
+          node-version-file: 'backend/package.json'
           cache: npm
           cache-dependency-path: backend/package.json
       - name: Install Dependencies

.github/workflows/frontend.yml

@@ -22,7 +22,7 @@ jobs:
       - name: Use Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: 20.x
+          node-version-file: 'frontend/package.json'
           cache: npm
           cache-dependency-path: frontend/package.json
       - name: Setup Pages

backend/.babelrc

@@ -0,0 +1,17 @@
+const pkg = require('./package.json');
+const nodeVersion = pkg.engines.node.replace('.x', '');
+
+module.exports = {
+  comments: false,
+  presets: [
+    ['@babel/preset-env', { modules: false, targets: { node: nodeVersion } }]
+  ],
+  plugins: ['@babel/plugin-transform-class-properties'],
+  env: {
+    test: {
+      presets: [
+        ['@babel/preset-env', { targets: { node: 'current' } }]
+      ]
+    }
+  }
+};

backend/babel.config.js

@@ -6,6 +6,10 @@
   "author": "Sebastian Messmer",
   "license": "UNLICENSED",
   "private": true,
+  "engineComment": "When updating node version, also update runtime in serverless.yml",
+  "engines": {
+    "node": "20.x"
+  },
   "scripts": {
     "test": "jest",
     "test:watch": "jest --watch",

backend/package.json

@@ -2,6 +2,9 @@
   "name": "cryfs-web-frontend",
   "version": "0.0.1",
   "description": "",
+  "engines": {
+    "node": "20.x"
+  },
   "scripts": {
     "dev": "next",
     "build": "next build",