FIX: loan summary zero values after navigation

[MrTob]

Issue (#1645)

After creating a loan, the "original principal" and "monthly payment" fields display correct values initially. However, after navigating away and returning to the overview tab, these values incorrectly show as 0.00.

Root Cause

The original_balance method in the Account model was prioritizing the first chronological balance record even when it had a zero value, instead of falling back to the current balance. This caused the loan calculations to use 0 as the base value after the initial page load.

Solution

Modified the original_balance method in the Account model to handle zero-value balance records more appropriately.
The change ensures that:

• If the first balance record exists but is zero, use the current balance
• If the first balance record has a non-zero value, use that
• If no balance record exists, fall back to current balance

[zachgoll]

Thanks for tackling this! I think your solution here would fix the summary card, but it will create inconsistencies with the historical graph.

In this case, a user will see a "start balance" of say, "$50,000", but then on the graph, the first value will still show as $0 leading to potential confusion.

I outlined a potential solution here that would make the initial values more appropriate for a loan-type account: #1612 (comment)