docs: add flashcard app design spec

Initial design for a single-user local flashcard webapp with hierarchical
lessons, Leitner-based spaced repetition, Excel import/export, and
animated practice UI.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,11 @@
+node_modules/
+dist/
+build/
+.DS_Store
+.env
+.env.local
+data/*.db
+data/*.db-*
+coverage/
+.vite/
+*.log