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/package.json

@@ -46,16 +46,13 @@
       "out/**"
     ],
     "asarUnpack": [
-      "**/node_modules/better-sqlite3/**"
+      "node_modules/better-sqlite3/**/*"
     ],
     "extraResources": [
       {
         "from": "../scripts/schema.sql",
         "to": "scripts/schema.sql"
       }
-    ],
-    "asarUnpack": [
-      "node_modules/better-sqlite3/**/*"
     ]
   },
   "dependencies": {
@@ -70,5 +67,6 @@
     "vite": "^6.0.0",
     "vue": "^3.4.0",
     "vue-router": "^4.3.0"
-  }
+  },
+  "type": "module"
 }