Fix “+” Button to Open Input Modal

Description:
The "+" button currently has no clear behavior in the UI. It’s expected to add a new transaction.

Proposed Solution:

Connect the button to open a modal/bottom sheet with input fields for description, amount, and category.

Ensure the modal contains validation for all fields.

Benefits:

Adds clarity to the add-entry process.

Streamlines user workflow.

Start by running the Flutter app and locating the currently inactive “+” button and the transaction-entry UI. Confirm that tapping it opens a modal or bottom sheet with description, amount, and category fields, each validated before a transaction can be added.

Written by the indexing model from the issue text.