a11y: add aria-label to Sidebar nav

[luke-ward88]

Background

Screen reader users rely on aria-label (or aria-labelledby) to distinguish between multiple <nav> landmarks on a page. The sidebar's <nav> in frontend/src/components/layout/Sidebar.tsx had no label, so it was announced as a generic "navigation" region -- indistinguishable from any other nav on the page.

Changes

Added aria-label to the sidebar's <nav> element, using the existing isChinese flag from useReviewLocale to provide a localized label:

<nav
  aria-label={isChinese ? '主导航' : 'Main navigation'}
  className="flex flex-col gap-1 p-2 sm:p-4"
>

Before

Screen reader announces: "navigation landmark" (no way to tell which nav).

After

Screen reader announces: "Main navigation landmark" (or "主导航" in Chinese).

Notes

• No other files touched.
• The isChinese flag is already destructured in the component, so no new hook calls needed.
• Follows the same pattern used by PublicLayout's header nav which already has an aria-label.