chore: update Node.js version to 20.19.0 across project files

- Updated the Node.js version in .node-version and package.json to 20.19.0.
- Revised README.md and docs/dev.md to reflect the new minimum Node.js requirement.
- Adjusted setup-node action to use the version specified in .node-version for consistency.

.github/actions/setup-node/action.yml

@@ -3,10 +3,10 @@ description: "Install pnpm and setup Node.js with pnpm cache (fixed versions)"
 runs:
   using: "composite"
   steps:
-    - name: Setup Node.js 20.12.0
+    - name: Setup Node.js
       uses: actions/setup-node@8f152de45cc393bb48ce5d89d36b731f54556e65 # v4.0.0
       with:
-        node-version: 20.12.0
+        node-version-file: '.node-version'
 
     - name: Setup pnpm
       shell: bash