fix(frontend): require node 20 for electron dev

[anirudh5harma]

Closes #412

Problem

The Electron frontend is effectively Node-20-only today, but contributors are
not told that up front. On Node 24/26, upstream Electron/Electron Forge install
failures can leave node_modules/electron half-installed, and the break only
surfaces later when npm run dev fails with Electron failed to install correctly.

Our desktop/frontend workflows already run on Node 20, so this is local-dev
friction caused by missing guardrails rather than an app-specific code path.

Change

• add engines.node = 20.x to frontend/package.json
• add a preinstall check that fails fast on unsupported Node versions with
  clear remediation steps
• add frontend/.nvmrc
• document the Node 20 requirement and the reason for it in the root README

Test

cd frontend
export NVM_DIR="$HOME/.nvm"
. "$NVM_DIR/nvm.sh"

nvm use 20
npm test

Result:

30 passed (30)
303 passed (303)

Unsupported-node guard:

node frontend/scripts/check-node-version.mjs

Verified:

• Node 26 fails immediately with the new explanatory message
• Node 20 passes the guard
• a fresh install on Node 26 now fails before a partial Electron install can
  corrupt node_modules

Notes

npm run typecheck still reports the pre-existing forge.config.ts typing
error around osxNotarize; this change does not modify that area.