feat(ui): unify header and implement navigation drawer (Task 17)

- Redesign top header into clean, uncluttered layout with hamburger drawer trigger, clickable logo, central SRS focus badge, theme toggle and profile avatar
- Implement unified responsive Navigation Drawer for both desktop and mobile with touch swipe-to-dismiss, backdrop blur, keyboard Esc support and background scroll lock
- Streamline mobile bottom tab bar to 4 core actions (Dictionary, SRS, Reader, Menu) with Menu opening the unified drawer
- Synchronize real-time SRS due badges and dictionary word statistics between header, bottom tabs, and drawer
- Eliminate obsolete separate mobile-menu-modal in favor of unified drawer
- Bump Service Worker cache version to notes-v5.7

app/static/sw.js

@@ -1,7 +1,7 @@
 // Service Worker for Notes English PWA (Stage 2)
 // Strategy: Cache-first for static assets, Network-first for API calls.
 
-const CACHE_NAME = 'notes-v5.6';
+const CACHE_NAME = 'notes-v5.7';
 const OFFLINE_URL = '/';
 
 const PRECACHE_URLS = [