The `electron:dev` script requires `dist-electron/main.js` to exist,
but the README does not mention running `node scripts/build-electron.mjs`
beforehand. Without this step, `electron .` fails with:
"Cannot find module '/path/to/dist-electron/main.js'"
Add the build command to both the Quick Start and Development sections
in all three README files (EN, CN, JA).