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/electron.vite.config.ts

@@ -24,6 +24,13 @@ export default defineConfig({
   },
   preload: {
     plugins: [externalizeDepsPlugin()],
+    build: {
+      rollupOptions: {
+        // Electron sandbox does not support ESM preload — emit CJS index.js
+        // (main loads ../preload/index.js) even though the project is ESM.
+        output: { format: 'cjs', entryFileNames: '[name].js' },
+      },
+    },
   },
   renderer: {
     plugins: [vue()],