chore(electron): wire package.json scripts + main entry, gitignore build output

Follow-up to 218a584f - these edits (electron npm scripts, "main" field,
electron/electron-builder/electron-updater deps, dist-electron/**
gitignore) were made alongside that commit but got left unstaged when it
landed. No behavior change beyond what that commit already described.

.gitignore

@@ -38,6 +38,10 @@ yarn-error.log*
 # vercel
 .vercel
 
+# electron (see electron/, scripts/build-electron.mjs, electron-builder.config.js)
+/dist-electron/
+/dist-electron-builds/
+
 # typescript
 *.tsbuildinfo
 next-env.d.ts