main: bail at startup if we don't have an openai api key

src/main.rs

@@ -509,6 +509,12 @@ enum AudioRecordRequest {
 #[tokio::main]
 async fn main() {
     color_eyre::install().unwrap();
+
+    if option_env!("OPENAI_API_KEY").is_none() {
+        eprintln!("Error: OPENAI_API_KEY environment variable not set. The application will not function without it.");
+        return;
+    }
+
     let mut terminal: Terminal<CrosstermBackend<std::io::Stdout>> = ratatui::init();
 
     let (prediction_in, mut prediction_out) = tokio::sync::watch::channel(None);