feat-699-accessible-mobile-navigation-drawer

[EcstaceeLOR]

Summary

Reworks the mobile navigation into an accessible slide-in drawer for viewports below 768px.

Changes

• Uses the 768px (md) breakpoint so navigation does not wrap on tablet-sized screens.
• Adds a right-side drawer with backdrop and smooth open/close transition.
• Supports Escape to close and backdrop click to dismiss.
• Moves keyboard focus into the drawer, traps Tab navigation while open, and restores focus when it closes.
• Removes closed drawer links from the tab order.

Verification

• Reviewed keyboard, focus, and responsive behavior in the component implementation.
• Local typecheck/test execution is pending because the environment cannot complete dependency installation without its certificate chain; this repository currently exposes no PR checks.

Closes #699

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