Hide console window in release build

Add windows_subsystem = "windows" so the packaged app launches without a
console black box (debug builds keep the console for logs).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src-tauri/src/main.rs

@@ -1,3 +1,5 @@
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")]
+
 use ceramic_radioactivity::{calculate_sample, CalculationResult, SampleInput};
 
 #[tauri::command]