feat: explicit PWA update prompt with build stamp

- registerType 'prompt' + skipWaiting/clientsClaim false: new SW waits
  instead of silently hijacking running sessions
- main.ts registers via virtual:pwa-register; onNeedRefresh shows
  dismissible toast (i18n en/ru/cu) with Update button -> updateSW(true)
- No polling — update check only on navigation, zero extra traffic
- Settings About shows __BUILD_DATE__ build stamp for remote debugging

vite.config.ts

@@ -6,11 +6,10 @@ import { VitePWA } from 'vite-plugin-pwa';
 export default defineConfig({
   plugins: [
     tailwindcss(),
-    VitePWA({
-      registerType: 'autoUpdate',
+    VitePWA({      registerType: 'prompt',
       workbox: {
-        skipWaiting: true,
-        clientsClaim: true,
+        skipWaiting: false,
+        clientsClaim: false,
         globPatterns: ['**/*.{js,css,html,woff,png}'],
         globIgnores: ['**/data/**'],
         maximumFileSizeToCacheInBytes: 20 * 1024 * 1024,
@@ -66,5 +65,8 @@ export default defineConfig({
     outDir: 'dist',
     sourcemap: true,
   },
+  define: {
+    __BUILD_DATE__: JSON.stringify(new Date().toISOString().slice(0, 10)),
+  },
   publicDir: 'static',
 });