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

src/ui/settings-view.ts

@@ -319,6 +319,7 @@ export class SettingsView {
               ${t.settings.aboutLicense}
             </a>
           </p>
+          <p class="text-[10px] text-navy-light mt-2 opacity-70">build ${__BUILD_DATE__}</p>
         </div>
       </div>
     `;