Add Edit Functionality

[7174Andy]

This pull request introduces support for editing existing transactions in the expense tracker application. The main changes include enhancements to the repository to allow transaction updates, the addition of a new dialog for editing expenses in the GUI, and refactoring of the main window to integrate the new editing functionality and improve dialog management. Tests have also been added to ensure correctness of the update logic.

Repository enhancements:

• Added an update_transaction method to TransactionRepository to allow updating specific fields of a transaction by ID, and annotated return types for several repository methods for clarity. [1] [2] [3] [4]

GUI improvements:

• Introduced a new EditExpenseDialog in expense_tracker/gui/dialogs/edit_expense.py to provide a user interface for editing transaction details.
• Updated main_window.py to use the new EditExpenseDialog for editing transactions, and refactored dialog management with a reusable _open_dialog method to prevent multiple dialogs from stacking. [1] [2] [3] [4] [5]

Testing:

• Added a test for the new update_transaction method to ensure transactions are correctly updated and unchanged fields are preserved.