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.

package.json

@@ -6,7 +6,7 @@
   "description": "Explicit memory infrastructure for coding agents — a queryable SQLite evidence layer over local Claude Code and Codex history, plus human-approved durable memory.",
   "license": "AGPL-3.0",
   "scripts": {
-    "test": "node --test tests/*.test.mjs",
+    "test": "node --experimental-test-module-mocks --test tests/*.test.mjs",
     "typecheck": "tsc --noEmit",
     "lint": "eslint .",
     "build:core": "rm -rf dist && tsc -p tsconfig.build.json"