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-service.js

@@ -1,6 +1,7 @@
-const fs = require('fs');
-const os = require('os');
-const path = require('path');
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import chokidarModule from 'chokidar';
 
 const DEFAULT_PROJECTS_DIR = path.join(os.homedir(), '.claude', 'projects');
 const DEFAULT_DEBOUNCE_MS = 2000;
@@ -38,7 +39,7 @@ function createIndexerService({
       if (!name || name.endsWith('.jsonl') || name.endsWith('.json')) onChange(name);
     };
     for (const root of existingRoots) {
-      const watcher = (chokidar || require('chokidar')).watch(root, {
+      const watcher = (chokidar || chokidarModule).watch(root, {
         cwd: root,
         ignoreInitial: true,
         awaitWriteFinish: {
@@ -192,4 +193,4 @@ function createIndexerService({
   };
 }
 
-module.exports = { createIndexerService };
+export { createIndexerService };