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.

tests/app-indexer.test.mjs

@@ -6,7 +6,7 @@ import { tmpdir } from 'node:os';
 import { join } from 'node:path';
 
 const require = createRequire(import.meta.url);
-const { buildIndex } = require('../app/src/main/indexer.js');
+import { buildIndex } from '../app/src/main/indexer.js';
 const { DatabaseSync } = require('node:sqlite');
 
 class TestDatabase {