Refactor method names for clarity and consistency across the application

- Renamed `initialize_csv` to `_initialize_csv_file` in `DataManager` for better clarity.
- Updated method calls in `GraphManager` from `_create_toggle_controls` to `_create_chart_toggles` and `_on_toggle_changed` to `_handle_toggle_changed`.
- Changed method names in `MedTrackerApp` from `on_closing` to `handle_window_closing`, `add_entry` to `add_new_entry`, and `load_data` to `refresh_data_display`.
- Adjusted corresponding test method names in `TestMedTrackerApp` to reflect the new method names.
- Updated `UIManager` method names from `setup_icon` to `setup_application_icon` and adjusted related tests accordingly.

src/data_manager.py

@@ -11,9 +11,9 @@ class DataManager:
     def __init__(self, filename: str, logger: logging.Logger) -> None:
         self.filename: str = filename
         self.logger: logging.Logger = logger
-        self.initialize_csv()
+        self._initialize_csv_file()
 
-    def initialize_csv(self) -> None:
+    def _initialize_csv_file(self) -> None:
         """Create CSV file with headers if it doesn't exist."""
         if not os.path.exists(self.filename):
             with open(self.filename, mode="w", newline="") as file: