fix(ux): Chrome Audit Polish - Theme Toggle, Modal A11y, Safe Shortcut Target, and Day-0 Heatmap

[arvarik]

Chrome Browser Test Review & Audit Polish

Following an automated test review using headless Google Chrome 152 via the Chrome DevTools Protocol (CDP), this PR resolves findings related to accessibility, theme toggling, event handling safety, and first-time onboarding.

What's Changed:

1. Direct Theme Switcher in Navigation:
   • Added persistent theme toggle buttons with Sun/Moon icons in both the desktop Sidebar footer (#theme-toggle-btn) and MobileDrawer.
   • Properly labeled with dynamic aria-label and keyboard shortcut hints (⌘D).
   • Covered with new unit test in sidebar_mobile.test.jsx.
2. Global Modal Accessibility & Escape Key Handling:
   • Added global Escape key listeners and ARIA modal attributes (role="dialog", aria-modal="true", aria-label) across all Modal components and KeyboardShortcutsModal.
3. Safe Keyboard Event Target Handling:
   • Protected useKeyboardShortcuts, FlashcardView, and FlashcardReviewModal against synthetic/document event dispatches where e.target?.tagName is undefined or non-element.
4. Day-0 Study Heatmap Onboarding:
   • Added an encouraging empty-state onboarding tip to StudyHeatmap when zero study sessions exist, explaining how spaced repetition reviews populate the retention and streak calendar.

Test Results:

• Unit & Integration Tests: 262 passed (0 failing).
• ESLint: 0 errors, 0 warnings.
• Production Build: Clean build output.
• Headless Chrome CDP Browser Suite: 10/10 test scenarios executed cleanly with 0 console errors, 0 warnings, and 0 network failures.