Improve accessibility of sidebar menu

[jguo144]

Description

Improve accessibility of sidebar menu

• Focus management: When the sidebar is opened (Enter on the menu label or click), focus moves to the first menu link after the open animation.
• Tab order when closed: When the sidebar is closed, its links and buttons are removed from the tab order (tabindex="-1") and the panel is hidden from the accessibility tree (aria-hidden="true") so keyboard users don’t tab into off-screen content.
• Keyboard: Enter toggles the sidebar when the menu label is focused; Escape closes it and returns focus to the menu trigger when focus was inside the sidebar.
• A11y: Resolves the Axe “scrollable region must be keyboard accessible” issue by applying aria-hidden to .main-navigation when closed.

[orca-security-us]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca