feat(pwa): Progressive Web App support with offline study capability

No PWA support — no service worker, no web manifest. Students study on phones and commutes. Kolibri's value proposition is offline-first education. Anki works offline.

Add manifest.json, Workbox service worker, offline flashcard review with IndexedDB + background sync.

**Closes gap against:** Kolibri (offline-first), Anki (offline), Quizlet (offline on premium)

**Effort:** L