The previous mobile fix broke desktop behavior by only taking the last result.
This fix uses resultIndex properly while also tracking processed results to
prevent duplication on mobile browsers where resultIndex stays at 0.
- Track processed results using a Set to avoid duplication
- Still use resultIndex for proper accumulation on desktop
- Clear processed results when starting a new session
Fixes speech result duplication issue on mobile while restoring
proper accumulation behavior on desktop browsers.
