Add default handling and write functionality to `ConfigManager`

- Ensure missing or empty config values are replaced with defaults.
- Write updated configuration back to `config.toml` using `tomli_w`.

src/metar_navigate/utils/config_manager.py

@@ -54,3 +54,13 @@ class ConfigManager(metaclass=SingletonMeta):
 
         }
 
+        for key, default_value in default.items():
+            current_value = self.__data.get(key)
+            if current_value is None or current_value == "":
+                self.__data[key] = default_value
+            else:
+                self.__data[key] = current_value
+        with open(f"{self.config_dir}/config.toml", "wb") as f:
+            tomli_w.dump(self.__data, f)
+
+