Add polite screen reader announcements for scan phases

[Muawiya-contact]

Goal

Make scan progress updates friendly for screen readers.

The scan panel updates file counts, bytes, phase, and preview rows while scanning. Screen reader users should get useful status changes without being spammed every 150ms.

Suggested starting point

Scan UI lives in:

app/src/App.jsx

The scan panel currently renders status text like:

<p className="scan-status">Scanning safely… nothing is being changed</p>
<p className="progress-phase">{phase || "Walking files"}</p>

Consider adding a polite live region for major phase changes only:

<p className="sr-only" aria-live="polite">{phase}</p>

Please avoid announcing every file count tick.

Acceptance criteria

• Major scan phase changes are announced politely.
• Rapid file count updates are not noisy for screen readers.
• Adds a small test or stylesheet check if a helper class like .sr-only is added.

[LouisDeconinck]

I'll take this one — planning to add a visually-hidden aria-live="polite" region to the scan panel that announces phase transitions only (not the per-tick file counts), plus a .sr-only helper and matching tests.