[F5:EC2] Expense Category Tab Container

[timebox: , complexity: ]

As a user, when I am looking at a given category, I want to see only the expenses that have been included in that category so that I can see those expenses at a glance.

The issue does not name any files, tests, or entry points. Start by locating the expense category tab and the expense-listing logic, then verify how category membership is represented. Done means opening a category shows only the expenses included in that category.

Written by the indexing model from the issue text.