refactor: Create main.py as entry point, implement on close

This commit creates a dedicated entry point `main.py`, instantiates
the application components, and handles the WM_DELETE_WINDOW protocol.

- Removed window creation and mainloop from previous file.
- Created a dedicated entry point with basic error handling.
- Implemented an on_closing protocol to properly close db and exit application
- Implemented the loading of database objects.

main.py

@@ -0,0 +1,22 @@
+import sys
+from database import Database
+from pdf_exporter import PDFExporter
+from ui.main_window import MainWindow
+
+
+def main():
+    db = Database()
+    pdf_exporter = PDFExporter()
+    app = MainWindow(db, pdf_exporter)
+
+    def on_closing():
+        db.close()
+        app.destroy()  # Properly destroy the Tkinter window
+        sys.exit()
+
+    app.protocol("WM_DELETE_WINDOW", on_closing)  # Handle window closing
+    app.mainloop()
+
+
+if __name__ == "__main__":
+    main()