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.

.gitignore

@@ -7,3 +7,4 @@ release/
 .dev.docs
 .claude/
 dist/
+app/out/