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/session-live.test.mjs

@@ -5,7 +5,7 @@ import {
   createSessionLiveState,
   consumeSessionDirty,
   noteSessionUpdated,
-} from '../app/renderer/src/session-live.mjs';
+} from '../app/src/renderer/src/session-live.mjs';
 
 test('session live state marks non-visible updated sessions as dirty', () => {
   const live = createSessionLiveState();