refactor(app): migrate to ESM; preload emitted as CJS for sandbox (Phase 5d-3b)

App source (main/preload/worker/renderer) -> ESM; app is now type: module;
__dirname via import.meta.url; worker spawned with type module. Preload is built
as CJS (electron-vite output format) because the sandboxed renderer does not
support ESM preload; main loads ../preload/index.js. Removed dead imports
(nativeImage, readline) and the obsolete scripts/dev.js.

Tests: 4 app tests require->import; app-main-settings rewritten with node:test
mock.module + dynamic import (replacing CJS Module._load mocking); test script
adds --experimental-test-module-mocks. electron-vite build clean, 119/119, and
npm run dev verified: app launches, preload bridges IPC, data loads.

app/src/main/indexer-worker.js

@@ -1,5 +1,5 @@
-const { parentPort } = require('worker_threads');
-const { buildIndex } = require('./indexer');
+import { parentPort } from 'node:worker_threads';
+import { buildIndex } from './indexer.js';
 
 parentPort.on('message', ({ id, args }) => {
   try {