feat: add slide-in mobile menu with focus-trap to landing nav bar (Closes #876)

[waterWang]

Summary

Adds a slide-in mobile menu with focus-trap to components/landing/landing-page-nav-bar.tsx for viewports below the md breakpoint (768px).

Previously the landing nav bar's mobile panel was a basic centered dropdown that overflowed/wrapped awkwardly under 768px. This replaces it with a proper right-side slide-in drawer.

Changes

• Slide-in drawer: Fixed panel that slides in from the right below md breakpoint, with a dimmed backdrop overlay
• Focus trap: Tab/Shift+Tab cycle stays within the open drawer; initial focus moves into the drawer on open
• Escape to close: Pressing Escape closes the drawer
• Focus return: Focus returns to the hamburger trigger button when the drawer closes
• Route change close: Drawer closes automatically on usePathname change
• Outside click close: Clicking the backdrop overlay closes the drawer
• Body scroll lock: Body scroll is locked while the drawer is open
• ARIA: role="dialog", aria-modal="true", aria-expanded, aria-controls, dynamic aria-label on the toggle
• Keyboard visible focus: focus-visible:ring on all interactive elements for WCAG 2.1 AA

Files changed

• components/landing/landing-page-nav-bar.tsx — slide-in drawer with focus management
• components/landing/landing-page-nav-bar.test.tsx — new tests covering open/close, focus trap, Escape, route change, outside click, and scroll lock

Tests

Added landing-page-nav-bar.test.tsx with 13 tests covering:

• Toggle open/close + aria-expanded state
• role="dialog" / aria-modal="true" attributes
• Tab/Shift+Tab focus wrapping within the drawer
• Focus return to the trigger button on close (Escape and button click)
• Close on route change and outside click
• Body scroll lock

Accessibility

WCAG 2.1 AA: keyboard navigable, focus trap, focus return, visible focus rings, ARIA dialog semantics, and aria-expanded/aria-controls on the trigger.

Closes #876

[vercel]

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