build(app): migrate to electron-vite (main/preload/renderer), keep electron-builder (Phase 5d-3a)

Restructure app into src/{main,preload,renderer}; electron.vite.config.ts builds
all three (each main module its own input so CJS requires + the indexer worker
resolve; better-sqlite3 externalized; Vue plugin for renderer). main/index.js
paths updated for the out/ layout + ELECTRON_RENDERER_URL. Still JS/CJS — TS+ESM
and core consumption are the next stages. Verified: npm run dev launches clean;
electron-vite build succeeds; root suite 119/119.

app/src/main/indexer-worker.js

@@ -0,0 +1,17 @@
+const { parentPort } = require('worker_threads');
+const { buildIndex } = require('./indexer');
+
+parentPort.on('message', ({ id, args }) => {
+  try {
+    const result = buildIndex(args || {});
+    parentPort.postMessage({ id, result });
+  } catch (error) {
+    parentPort.postMessage({
+      id,
+      error: {
+        message: error.message,
+        stack: error.stack,
+      },
+    });
+  }
+});