feat(auth): implement multi-account switcher and encrypted saved accounts UI

- Add Web Crypto AES-GCM 256-bit token encryption in account-crypto.ts
- Implement account switcher state store in account-store.ts with LRU eviction and active session rotation
- Add Google-style 1-tap Account Chooser on LoginPage with fast switch and re-auth fallback
- Add SavedAccountsCard on ProfilePage and account indicator on HomePage
- Add AccountTransitionOverlay for smooth switching animation
- Intercept and synchronize refreshed tokens across saved accounts in api-client.ts
- Add full i18n support in en.json and vi.json