chore(deps): npm update + Node 22 LTS engines

Brings the dependency tree up to the latest in-range versions of every
package: codemirror ^6.43.13, dompurify ^3.4.16, mermaid stays at
^11.17.2 (v12 needs a rendering QA pass before bumping), highlight.js
^11.12.0, jszip ^3.10.2, katex ^0.18.10, prettier ^3.9.9, etc.

Also pins the dev Node version to >=22 LTS via the engines field.
Currently running on Node 22.22.1.

Deliberately deferred (would each need a QA pass):

  - Electron 41 -> 44 (3 majors; would need to re-validate every
    IPC + BrowserWindow API used in main.js / preload.js / the
    standalone windows).
  - Mermaid 11 -> 12 (major rendering pipeline change).
  - Marked 17 -> 18 (markdown renderer rewrite).
  - ESLint 9 -> 10 (flat-config breaking changes).
  - pdfjs-dist 5 -> 6 (worker pipeline rework).
  - electron-store 10 -> 11 (storage backend swap).

In-range updates verified by the full test suite (1386/1386).

Amit Haridas

package.json

@@ -4,6 +4,9 @@
   "description": "Professional Markdown editor and universal file converter with PDF editing, batch processing, and syntax highlighting",
   "main": "src/main.js",
   "desktopName": "MarkdownConverter",
+  "engines": {
+    "node": ">=22.0.0"
+  },
   "scripts": {
     "start": "electron .",
     "test": "cross-env NODE_OPTIONS=--experimental-vm-modules jest",