docs: add UI detail polish design spec

Spec covering radius/spacing tokens, interaction feedback (hover/active/
focus-visible), and a mobile workspace-toolbar wrapping fix, based on
live screenshot review of the running app.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -27,3 +27,6 @@ dist-ssr
 # Backend data and secrets
 data/teaching-books.db
 .env
+
+# Brainstorming visual companion
+.superpowers/