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/js/modules/srs.js

@@ -115,6 +115,7 @@ export function initSrs() {
     // Stats & Heatmap Modal
     const statsOpenBtn = document.getElementById('stats-open-btn');
     const mobileStatsBtn = document.getElementById('mobile-stats-btn');
+    const drawerNavStats = document.getElementById('drawer-nav-stats');
     const statsModal = document.getElementById('stats-modal');
     const closeStatsModalBtn = document.getElementById('close-stats-modal-btn');
     const closeStatsBtn = document.getElementById('close-stats-btn');
@@ -646,6 +647,7 @@ export function initSrs() {
     // -----------------------------------------------------------------------
     if (statsOpenBtn) statsOpenBtn.addEventListener('click', openStatsDashboard);
     if (mobileStatsBtn) mobileStatsBtn.addEventListener('click', openStatsDashboard);
+    if (drawerNavStats) drawerNavStats.addEventListener('click', openStatsDashboard);
     closeStatsModalBtn.addEventListener('click', () => closeModal(statsModal));
     closeStatsBtn.addEventListener('click', () => closeModal(statsModal));