fix(build): align electron-builder with Electron 42 and remove version drift

[ThomasHalwax]

Summary

PR #117 bumped the electron devDependency to 42.2.0 but left electron-builder.yml pinned at electronVersion: 38.8.0. That field is the single source of truth for both packaging and the webpack build target (webpack.config.js derives the electronXX-* target from it), so release builds would have bundled Electron 38 — leaving the audit advisories #117 set out to close present in shipped artifacts.

This PR repairs the drift and removes the duplication that caused it.

Impact

No released artifact is affected: the Electron 42 bump is in no tagged release (the latest, v3.3.0, predates it by ~2 months). This lands before the next release.

Changes

• electron-builder.yml — drop electronVersion. With the field absent, electron-builder auto-detects the version from the installed electron dependency.
• webpack.config.js — derive the build target from require('electron/package.json').version instead of parsing electron-builder.yml; drop the now-unused fs and yaml requires.
• package.json — pin node-abi to ^4.31.0 via overrides. Bumping the Electron version surfaced that node-abi@4.26.0 (under @electron/rebuild) cannot detect the Electron 42 ABI, which fails electron-builder install-app-deps.

Result

The electron dependency is now the single source of the Electron version. Both packaging and the webpack target read the installed package, so they cannot drift apart again.

Validation

• electron-builder install-app-deps auto-detects electronVersion=42.2.0 and rebuilds leveldown for it
• webpack compiles with the electron42.2-* target
• npm run lint clean · 332 tests passing · npm audit 0 findings