fix(a11y): announce FFmpeg loading and export status via aria-live region

[sweetylearner-max]

Related Issue

Closes #168

Description of Changes

• Added announcement state and prevStatusRef to track status transitions in ExportOverlay.tsx
• Added a <div role="status" aria-live="polite" className="sr-only"> region that announces:
  • "Loading video processing engine, please wait..." when the engine starts loading
  • "Export started. Processing your video locally." when export begins
• Announcement clears when the overlay is dismissed

Acceptance Criteria

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

Type of Change

• Accessibility improvement

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