fix(MobileMenuToggle): trap keyboard focus inside the mobile menu

[timothyjordan]

Description

Fixes #2697.

When the mobile menu is open on narrow viewports, Tab navigation currently escapes the menu after the last focusable element (the theme switcher) and lands on the underlying page content.

This PR extends the existing StarlightMenuButton custom element with a keydown handler that wraps Tab / Shift+Tab focus between the first and last visible focusable elements inside the surrounding <nav> while the menu is expanded (body[data-mobile-menu-expanded]).

The trap is scoped to the same <nav> already used for the Escape listener and is gated on the existing data attribute, so there is no behavioural change while the menu is closed.

Test plan

• Added Playwright e2e coverage in packages/starlight/__e2e__/mobile-menu.test.ts:
  • Tab from the last focusable wraps to the first.
  • Shift+Tab from the first wraps to the last.
  • Trap does not engage while the menu is closed.
  • Escape still closes the menu and restores focus to the toggle (regression check).
• All existing unit tests (519) and e2e tests pass locally.

Changeset

Patch bump for @astrojs/starlight included.

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🦋 Changeset detected

Latest commit: cb0c019

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@astrojs/starlight	Patch

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