Bug in test_playground/intermediate/boss.py

[narrative26]

Bug Description
Two helper functions normalize_user_id and compute_tax are defined but never called anywhere in the codebase, leading to incorrect cart file handling and missing tax calculation at checkout.

To Reproduce
Steps to reproduce the behavior:
For normalize_user_id:

1. Launch the app
2. Type Student1 in the User ID field and add items to cart
3. Switch user, type student1 and add items
4. Notice two separate cart files are created: cart_Student1.json and cart_student1.json, instead of one

For compute_tax:

1. Launch the app
2. Add any items to cart
3. Click Checkout
4. Notice the total shown and saved to CSV has no tax applied

Expected behavior
normalize_user_id should be called when creating a CartManager so that Student1, STUDENT1, and student1 all map to the same cart file
compute_tax should be called in checkout() so that the final bill includes tax.

Additional context
Both functions have the comment # helper function for practice (UI does not depend on this) suggesting they were intentionally left unconditioned as a practice exercise, but in a real deployment both would cause data integrity issues.