Add Confirmation Flash Message for Extravaganza Interest Selection

Issue Description

When students scan the Extravaganza QR code and select or deselect an interest checkbox, the change is automatically submitted. However, there is currently no visible confirmation that the update was successfully saved.

Add a flash message that appears after each checkbox update to reassure students that their selection was recorded. Since this page will primarily be used on phones during Extravaganza, the flash message should be mobile-friendly and display clearly on smaller screens.

Expected Behavior
A confirmation flash message appears after each successful checkbox update.
The message clearly indicates that the student's selection was saved.
The flash message works properly when checking or unchecking an option.
The flash message is responsive and easy to read on mobile devices.

UI:

Start at the Extravaganza interest-selection page and trace the checkbox submission flow for both checking and unchecking. Add a clear, mobile-friendly confirmation after each successful update, then verify the message is readable on small screens and appears only after the selection is saved.

Written by the indexing model from the issue text.