Fixes#2793.
## Problem
IndexaCapitalAccount::ActivitiesProcessor#normalize_cash_amount used the inverted amount-sign convention relative to Sure's core conventions.
- Outflows (WITHDRAWAL, TRANSFER_OUT, FEE, TAX) were stored as negative amounts.
- Inflows (CONTRIBUTION, TRANSFER_IN, DIVIDEND, DIV, INTEREST) were stored as positive amounts.
Sure requires asset account inflows to be stored as negative amounts (-amount.abs) and outflows as positive amounts (amount.abs).
## Fix
Flip signs in IndexaCapitalAccount::ActivitiesProcessor#normalize_cash_amount to align with Sure's sign convention and sibling processors (SnaptradeAccount::ActivitiesProcessor).
## Test
Added unit tests in test/models/indexa_capital_account/activities_processor_test.rb covering cash inflows, outflows, transfers, fee, label mappings, and empty payloads. All tests pass (39/39 for indexa_capital_account).
Co-authored-by: erkdgn <erkdgn@users.noreply.github.com>
