* feat: Make Whisper tiny the default engine and download during install
- Changed default engine from 'vosk' to 'whisper' in config_manager.py
- Changed default model_size from 'small' to 'tiny' in config_manager.py
- Added install_whisper_model() function to install.sh that downloads
the Whisper tiny model (~75MB) during installation
- Whisper model is downloaded when Whisper package is installed
(in non-interactive mode or with --with-whisper flag)
- Updated installation summary to show Whisper model status
- Updated documentation (README.md, INSTALL.md, SECURITY.md, USER_GUIDE.md)
to reflect Whisper tiny as the default engine
This provides a better out-of-box experience: users can install and
immediately use Vocalinux without any additional downloads or prompts.
* test: Update test to reflect whisper as default engine
