fix: add focus trap and accessibility tests for mobile menu (Closes #512)

[waterWang]

Summary

Adds a focus trap to the Navbar's mobile menu using the existing useFocusTrap hook, ensuring keyboard navigation stays within the menu while it is open. Also adds tests to verify the focus trap behavior.

Changes

• Navbar.tsx: Import and attach useFocusTrap to the mobile menu container. When the menu opens, focus is trapped inside; pressing Escape closes the menu and returns focus to the hamburger toggle.
• Navbar.test.tsx: Added two new tests:
  1. Focus moves into the mobile menu when it is opened.
  2. Escape closes the menu and returns focus to the toggle button.

Acceptance Criteria

• Focus is trapped inside the mobile menu while open
• Escape closes the menu and returns focus to the toggle button
• All existing tests still pass (7/7)
• No new TypeScript errors introduced

Guidelines

• Minimum 95% test coverage ✓
• Timeframe: 96 hours