[Accessibility] Add 'Loading' state announcement when FFmpeg engine loads

[magic-peach]

Overview

When the FFmpeg engine is loading, announce this to screen reader users so they know to wait and the app hasn't frozen.

Implementation

<div role="status" aria-live="polite" className="sr-only">
  {status === 'loading-engine' && 'Loading video processing engine, please wait...'}
</div>

Acceptance Criteria

• 'Loading engine' announced to screen reader
• When complete, 'Export started' announced
• Uses polite live region (doesn't interrupt)

[aksharsawhney74-rgb]

Hi @magic-peach ,

I would love to work on this performance optimization issue for GSSoC '26!
Could you please assign this to me?

Thanks,
Akshar

[magic-peach]

@aksharsawhney74-rgb please star the repo