Add NeetCode 250 reference page with pattern templates

Topic → template → questions organization. Each of the 18 topics has a
memorizable pseudocode template (plus Python/JS skeletons) and lists the
questions that template solves. Problem titles link to the in-app problem
page when available, falling back to an external LeetCode link.

Open accordion state persists in localStorage (shared hook also applied to
the DSA Reference page) so navigating away and back preserves context.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: niamleeson