Function resolve_whisper_model_path 

pub fn resolve_whisper_model_path(
    config: &AppConfig,
) -> Result<PathBuf, AppError>

Resolve the path to the local Whisper model file.

Checks (in order):

1. config.voice.local_model_path if set
2. GESTURA_WHISPER_MODEL environment variable
3. Default search directories (~/.gestura/models/, ./models/)

Returns an error with an actionable message if no model is found.