Add docstrings to unpaid_invoices.py functions over 3 LOC

[Copilot]

Added Python docstrings to all functions in unpaid_invoices.py exceeding 3 lines of code, improving code documentation and maintainability.

Changes

• 14 functions documented with descriptive docstrings explaining purpose, behavior, and side effects
• 6 functions unchanged (≤3 LOC per requirement)

Each docstring includes:

• Brief summary of function purpose
• Description of key operations and data flow
• Notes on side effects (database operations, UI updates, external calls)

Example

def populate_unpaid_invoices(self):
    """Load and display all unpaid invoices in the treeview.
    
    Queries the database for all posted, unpaid, non-canceled invoices
    and displays them in the treeview. Calculates and updates the total
    unpaid invoice amount and the accounts receivable balance. Preserves
    the current selection if it still exists after refresh.
    """
    # ... existing implementation

No functional changes—documentation only.

Original prompt

add code comments to unpaid_invoices.py for all functions over 3 LOC

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