Fix/SELL fee added to cash balance instead of subtracted

When creating a SELL activity with a non-zero fee, the cash balance on
the account was credited by (quantity * unitPrice) + fee instead of
(quantity * unitPrice) - fee, inflating the cash balance by 2x the fee.

The previous logic added the fee to the gross magnitude before negating
the entire amount for BUY/FEE. SELL (and DIVIDEND/INTEREST/LIABILITY
when carrying a fee) therefore received +gross + fee.

Compute the signed gross amount first, then subtract the fee
unconditionally — fees always reduce cash regardless of activity
direction.

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Fixed
+
+- Fixed the cash balance computation for `SELL` activities to subtract the fee instead of adding it (the previous logic inflated the account's cash balance by twice the fee)
+
 ## 3.3.0 - 2026-05-14
 
 ### Added

apps/api/src/app/activities/activities.service.ts

@@ -214,15 +214,15 @@ export class ActivitiesService {
     });
 
     if (updateAccountBalance === true) {
-      let amount = new Big(data.unitPrice)
-        .mul(data.quantity)
-        .plus(data.fee)
-        .toNumber();
+      let amount = new Big(data.unitPrice).mul(data.quantity).toNumber();
 
       if (['BUY', 'FEE'].includes(data.type)) {
         amount = new Big(amount).mul(-1).toNumber();
       }
 
+      // Fees always reduce cash regardless of the activity direction
+      amount = new Big(amount).minus(data.fee).toNumber();
+
       await this.accountService.updateAccountBalance({
         accountId,
         amount,