Announce update status and progress to assistive technology

[tsouth89]

Problem

The update banner changes from available to downloading, ready, and failed, but it has no live-region/status semantics. Its progress bar is visual-only, and failures are not announced as alerts.

Where

apps/desktop-tauri/src/components/UpdateBanner.tsx

Expected behavior

• Announce non-disruptive state changes with role="status" and/or an appropriate aria-live region.
• Give download progress semantic progressbar values.
• Announce failures with alert semantics.
• Avoid repeatedly announcing unchanged text during progress updates.

Tests

Add focused assertions for available, downloading, ready, and failed states, including progress values.