Add Escape key handler for mobile nav and ToC

[braboj]

Summary

CLAUDE.md section 2.8 states "Navigation menu MUST close on Escape and restore focus." No keydown event listener for Escape exists in the hamburger menu or mobile ToC overlay scripts.

Keyboard-only users who open these elements have no keyboard shortcut to dismiss them.

Acceptance criteria

• Pressing Escape closes the mobile hamburger menu and restores focus to the toggle button
• Pressing Escape closes the mobile ToC overlay and restores focus to the FAB

Found by

360-degree analysis (Value + Quality categories), 2026-04-29.