fix(core): expose aria-expanded and aria-controls on MobileNav toggle

[bhamodi]

Summary

MobileNavToggle rendered a Button with an aria-label and onClick, but exposed no aria-expanded and no aria-controls (packages/core/src/MobileNav/MobileNavToggle.tsx:69). Screen-reader users had no way to tell whether the nav drawer was currently open, nor which element the hamburger button controls.

Making aria-controls resolve required a shared id, which did not exist: the AppShell mobile context (useAppShellMobile()) exposed no drawer id, and the MobileNav <dialog> had no id at all. This wires that up end to end:

• AppShell generates a stable useId() and publishes it on the mobile context as mobileNavId.
• MobileNav applies that id to its <dialog> (falling back to a locally generated useId() when used standalone outside AppShell).
• MobileNavToggle sets aria-expanded={isMobileNavOpen} and aria-controls={mobileNavId}.

aria-controls is set unconditionally here (unlike the conditionally-mounted region in #3719): the native <dialog> stays in the DOM whenever the layout is below the mobile breakpoint -- it is mounted via React <Activity mode="hidden"> (or a plain fragment fallback) and simply hidden when closed -- so the referenced id always resolves. Purely additive; no behavior or visual change.

Changes

• MobileNav/MobileNavToggle.tsx: forward aria-expanded + aria-controls from context onto the toggle button.
• MobileNav/MobileNav.tsx: apply the shared mobileNavId (with a standalone useId() fallback) as the <dialog> id.
• AppShell/AppShell.tsx + AppShell/AppShellMobileContext.tsx: generate and publish mobileNavId on the mobile context.
• AppShell/useAppShellMobile.doc.mjs: document the new mobileNavId return field.
• MobileNavToggle showcase blocks + SideNav.test.tsx fixtures: supply mobileNavId where the context value is constructed by hand.

Test plan

• node_modules/.bin/vitest run --root . packages/core/src/MobileNav packages/core/src/AppShell -- 55 pass (4 files), including three new tests in MobileNavToggle.test.tsx (all confirmed failing before the fix):
  • exposes aria-expanded="false" when the drawer is closed
  • exposes aria-expanded="true" after opening the drawer
  • references the nav drawer via aria-controls that resolves to the dialog -- asserts the toggle's aria-controls resolves via document.getElementById to the <dialog> element.
• node_modules/.bin/vitest run --root . packages/core/src/SideNav packages/core/src/TopNav -- 151 pass (they render MobileNav in drawer mode).
• node_modules/.bin/vitest run --root . packages/core -- 3848 pass (full core suite, incl. the docs contract test).
• node_modules/.bin/tsc --project packages/core/tsconfig.json --noEmit -- clean.
• node_modules/.bin/eslint on changed files -- clean.

Notes

Found during a broader a11y audit of core components; scoped to one fix per PR. Button forwards arbitrary aria-* props to the underlying <button> (the same {...props} spread the existing behavior relies on), so aria-controls reaches the DOM. Standalone MobileNav (outside AppShell) still gets a valid unique <dialog> id via its own useId().

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