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.js

@@ -1,7 +1,10 @@
-const fs = require('fs');
-const os = require('os');
-const path = require('path');
-const Database = require('better-sqlite3');
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import Database from 'better-sqlite3';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
 
 const TEXT_LIMIT = 10000;
 const DEFAULT_CLAUDE_DIR = path.join(os.homedir(), '.claude');
@@ -1161,7 +1164,7 @@ function buildIndex({
   }
 }
 
-module.exports = {
+export {
   buildIndex,
   writeHeartbeat,
   openIndexDb,