Add user creation API and enhance user preference handling in the frontend

Refactor explainer methods and enhance evaluation template with detailed CF approach explanations

Enhance evaluation API and frontend for collaborative filtering methods

Refactor recommendation page and templates

Fix SVD convergence and approach comparison data leakage

idk

Fix user-based CF NaN error and make constraint-based brand soft

Complete 3-approach evaluation comparison in API and UI

Initial implementation of TasteEngine: multi-approach recommender system with Flask web UI, evaluation suite, and explanation engine