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.

tests/app-indexer.test.mjs

@@ -6,7 +6,7 @@ import { tmpdir } from 'node:os';
 import { join } from 'node:path';
 
 const require = createRequire(import.meta.url);
-const { buildIndex } = require('../app/indexer.js');
+const { buildIndex } = require('../app/src/main/indexer.js');
 const { DatabaseSync } = require('node:sqlite');
 
 class TestDatabase {