feat(ui): UI overhaul — consolidates #305-#309 (keyboard shortcuts, perf monitor, toasts, theme, command palette, activity log, data export, mobile PWA, accessibility, i18n) (#620)

* feat(ui): add keyboard shortcuts, perf monitor, toast system, theme toggle, and WCAG accessibility

- Keyboard shortcuts overlay (press ? for help, 1-8 for tabs, T for theme, P for perf)
- Real-time performance monitor with FPS, memory, latency sparklines (draggable)
- Enhanced toast notification system with stacking, auto-dismiss, progress bars
- Dark/light theme toggle with localStorage persistence and system preference detection
- WCAG accessibility: skip-to-content link, ARIA roles/attributes on tabs and panels,
  arrow key navigation in tab bar, focus-visible outlines
- ESLint config for UI directory with security and quality rules

* feat(ui): add command palette, activity log, data export, fullscreen mode, connection status

- Command palette (Ctrl+K / Cmd+K) with fuzzy search across tabs and actions
- Activity log panel (L key) with real-time console interception, filters, resizable
- Data export utility (E key) for sensor data as JSON/CSV with dialog
- Fullscreen mode (F key / F11) for visualization tabs with exit button
- Connection status widget in header showing WebSocket state and reconnect

* feat(ui): add mobile hamburger nav, PWA support, and 40 unit tests

- Mobile hamburger navigation: slide-out drawer replacing tab bar on <768px,
  swipe-to-close, animated hamburger icon, auto-sync with tab manager
- PWA manifest + service worker: installable dashboard, offline shell caching
  (cache-first for static, network-first for API), auto-cleanup of old caches
- 40 unit tests for ToastManager, ThemeToggle, KeyboardShortcuts, PerfMonitor,
  TabManager - browser-based test runner at ui/tests/unit-tests.html
- PWA meta tags: theme-color, apple-mobile-web-app-capable, manifest link
- Icon generator page for creating PWA icons (ui/icons/generate.html)

* feat(ui): add URL routing, onboarding tour, idle detection, notification center

- Hash router: tabs are bookmarkable/shareable via URL (#demo, #sensing, etc.),
  syncs with TabManager, supports browser back/forward navigation
- Onboarding tour: interactive 6-step first-run walkthrough with spotlight
  highlighting, step indicators, skip/back/next controls, localStorage persistence
- Idle detection: pauses health polling and reduces CSS animations after 3 min
  of inactivity, resumes on user interaction, integrates with Page Visibility API
- Notification center: bell icon in header with unread badge, event history panel
  with mark-read/clear, persists across page views via sessionStorage

* feat(ui): add i18n (EN/PL), screenshot tool, settings panel, reduced motion, uptime clock

- i18n: English/Polish translations with auto-detection, language selector
  in header, data-i18n attributes on dashboard elements, localStorage persistence
- Screenshot tool (S key): captures active tab to clipboard or downloads PNG,
  flash effect, canvas rendering with watermark, fallback for tainted canvases
- Quick settings panel (gear icon): reduced motion toggle, high contrast mode,
  compact layout mode, health polling toggle, clear data, reset onboarding
- Uptime clock: current time + session duration in header
- prefers-reduced-motion: system-level and manual toggle, disables all
  animations and transitions for vestibular accessibility
- High contrast mode: WCAG AAA compliant colors for both light and dark themes
- Compact mode: condensed layout for dense information display

ui/components/TabManager.js

@@ -19,6 +19,33 @@ export class TabManager {
       tab.addEventListener('click', () => this.switchTab(tab));
     });
 
+    // Arrow key navigation within tab bar (WCAG)
+    const nav = this.container.querySelector('.nav-tabs');
+    if (nav) {
+      nav.addEventListener('keydown', (e) => {
+        const buttonTabs = this.tabs.filter(t => t.tagName === 'BUTTON' && !t.disabled);
+        const currentIndex = buttonTabs.indexOf(document.activeElement);
+        if (currentIndex === -1) return;
+
+        let nextIndex = -1;
+        if (e.key === 'ArrowRight' || e.key === 'ArrowDown') {
+          nextIndex = (currentIndex + 1) % buttonTabs.length;
+        } else if (e.key === 'ArrowLeft' || e.key === 'ArrowUp') {
+          nextIndex = (currentIndex - 1 + buttonTabs.length) % buttonTabs.length;
+        } else if (e.key === 'Home') {
+          nextIndex = 0;
+        } else if (e.key === 'End') {
+          nextIndex = buttonTabs.length - 1;
+        }
+
+        if (nextIndex >= 0) {
+          e.preventDefault();
+          buttonTabs[nextIndex].focus();
+          this.switchTab(buttonTabs[nextIndex]);
+        }
+      });
+    }
+
     // Activate first tab if none active
     const activeTab = this.tabs.find(tab => tab.classList.contains('active'));
     if (activeTab) {
@@ -36,14 +63,22 @@ export class TabManager {
       return;
     }
 
-    // Update tab states
+    // Update tab states and ARIA attributes
     this.tabs.forEach(tab => {
-      tab.classList.toggle('active', tab === tabElement);
+      const isActive = tab === tabElement;
+      tab.classList.toggle('active', isActive);
+      if (tab.hasAttribute('aria-selected')) {
+        tab.setAttribute('aria-selected', String(isActive));
+      }
     });
 
-    // Update content visibility
+    // Update content visibility and ARIA
     this.tabContents.forEach(content => {
-      content.classList.toggle('active', content.id === tabId);
+      const isActive = content.id === tabId;
+      content.classList.toggle('active', isActive);
+      if (content.hasAttribute('role')) {
+        content.setAttribute('aria-hidden', String(!isActive));
+      }
     });
 
     // Update active tab