fix(install): require Node >=20.19/22.12 for the desktop build

The "Build desktop app" install step failed with an opaque "exit code 1"
on machines with an old Node, and nothing in the logs explained it.

Reproduced: on Node 20.5.1, `npm run pack`'s `vite build` crashes with

  You are using Node.js 20.5.1. Vite requires Node.js version 20.19+ or 22.12+.
  SyntaxError: The requested module 'node:util' does not provide an
  export named 'styleText'

Vite 8 (rolldown) imports node:util.styleText, which doesn't exist before
Node 20.12, so the build dies before producing the app. The installer's
check_node / Test-Node accepted ANY pre-existing Node with no version
floor, so a too-old system Node was used for the build instead of the
bundled Node 22.

Add a version floor (^20.19 || >=22.12) to check_node (install.sh) and
Test-Node (install.ps1): a too-old system Node is replaced with the
Hermes-managed Node 22 LTS, and the desktop stage re-resolves Node so the
build always runs on a satisfying version. Declare the same range in
apps/desktop/package.json engines.

Verified: build succeeds on Node 22, fails on 20.5.1 with the error above;
the floor logic matches Vite's range across boundary versions (20.18/20.19,
21.x, 22.11/22.12).

apps/desktop/package.json

@@ -7,6 +7,9 @@
   "author": "Nous Research",
   "type": "module",
   "main": "electron/main.cjs",
+  "engines": {
+    "node": "^20.19.0 || >=22.12.0"
+  },
   "scripts": {
     "dev": "concurrently -k \"npm:dev:renderer\" \"npm:dev:electron\"",
     "dev:fake-boot": "cross-env HERMES_DESKTOP_BOOT_FAKE=1 HERMES_DESKTOP_BOOT_FAKE_STEP_MS=650 npm run dev",