Add DSA Pattern Lab — complete 15-set interactive study app

Single-file HTML app covering all 25 patterns of the FAANG prep atlas:
sliding window & prefix sum, arrays & hashing, two pointers, stack &
monotonic stack, binary search, linked list, trees, heap & top-K,
intervals & greedy, graphs, backtracking, union-find & trie, DP I-III,
design data structures, and odds & ends. Includes fold-shut solutions,
a step-through trace player, 99 flashcards, a 110-problem
spaced-repetition ladder, and an 82-question journal (localStorage).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: DeepCodeWork