fix(recording): save recording when captured window closes

[ignacio-sj]

Summary

• When recording a window that gets closed, the app now attempts to save the recording instead of discarding it
• Previously, any non-.userStopped stream error during recording would call assetWriter.cancel(), losing all captured data
• Now both code paths (user stop sharing + unexpected stream stop) call stopRecording(), which finalizes the file gracefully and falls back to cancel if finalization fails

Test plan

• Start recording a specific window
• Close that window while recording is active
• Verify the recording is saved to the output folder
• Verify the saved file is playable in QuickTime

[jsattler]

Thanks for fixing!