feat: add PWA update notifications and service worker update flow

[DeePrincipal-dev-lang]

feat: PWA update notifications and service worker update flow

Summary

Users were not receiving notifications when a new version of the app was available, requiring manual
refresh to get updates.

Changes

• public/sw.js – Service worker with skipWaiting, clientsClaim, and offline fallback
• src/utils/registerSW.ts – Registration utility that detects a waiting SW and triggers the update
  callback; auto-reloads on controllerchange
• src/components/UpdateBanner.tsx – Accessible update prompt banner (role="alert", aria-live="polite")
  with "Update Now" / "Later" actions using lucide icons
• src/components/pwa/AppUpdateManager.tsx – Refactored to use UpdateBanner, removing duplicated markup
• src/hooks/usePWA.tsx – Delegates to registerSW / applyUpdate utilities; removed manual event
  listener boilerplate

Update Flow

PWAManager registers SW on mount → registerSW detects waiting SW → updateAvailable = true →
UpdateBanner shown → user clicks "Update Now" → SKIP_WAITING posted → new SW activates → page reloads
automatically

Testing

• Install the app as a PWA
• Deploy a new version (bumps SW file)
• Confirm the update banner appears without manual refresh
• Click "Update Now" and confirm the page reloads with the new version

Closes #243

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