Add Dropdown or Autocomplete for Category Input

Description:
Currently, the category field allows free-text input, leading to inconsistent entries like "Food", "food", "new", or random test data. This makes filtering and analysis difficult.

Proposed Solution:

Replace the free-text input with a dropdown list of predefined categories (e.g., Food, Transport, Utilities, etc.)

Optionally implement an autocomplete field with suggestions based on existing user input.

Benefits:

Ensures category consistency.

Improves user experience.

Enables better filtering and reporting.

No files, tests, or entry points are named. Locate the expense-entry category field and inspect how its value is stored, filtered, and displayed; then verify that selecting a predefined category prevents inconsistent free-text values and that filtering still works.

Written by the indexing model from the issue text.