fix(electron): ship loginManager.js in the packaged app (#3292 regression) (#3334)

#3292 added electron/loginManager.js and a require("./loginManager") in
main.js but did not add it to electron-builder's build.files allowlist, so
the packaged app crashed at startup with "Cannot find module './loginManager'"
on the Linux/macOS smoke tests (v3.8.13 Electron release fragment).

Add loginManager.js to build.files, plus a regression test that asserts every
local require("./x") in the Electron entry points is shipped.

electron/package.json

@@ -51,6 +51,7 @@
     "files": [
       "main.js",
       "preload.js",
+      "loginManager.js",
       "sqlite-inspection.js",
       "package.json",
       "node_modules/**/*"