fix: Balance Sheet cumulative balance calculation

[le-anne]

Hi, I noticed this was a critical issue affecting the balance sheet report. I'm an accounting student and hobbyist developer and thought I'd like to take a try to debug.

Incorrect Balances - Resolves #815
Preciously, the balance sheet accounts were displaying incorrect amounts. Upon deeper investigation, I noticed it was the balance that should have been carried over from the previous year. The balance would reset at the beginning of the year.

Changes Implemented:
AccountReport.ts:
Corrected _getGroupedByDateGranges to accurately calculate and manage account balances.

I changed the concept of totalBalance to a runningBalance, which accurately reflects its cumulative balance across the sorted dates.

This also included adding an abstract get rootTypes() for TypeScript compilation.

Balance Sheet Report Filters:
Based On: Fiscal Year
Periodicity: Yearly
From Year: 2020
To Year: 2025

I created test journal entries and checked them display consistently across the selected criteria.

[akshayitzme]

@le-anne Thanks for raising this PR. Could you take a look at the failing checks ?