feat: build modular element detail page

docs/AI_USAGE.md

@@ -174,6 +174,19 @@
 - **Review:** The team refined the layout, colors, difficulty display, and
   accessibility semantics during review.
 
+## Element detail page (`src/components/elements/`, `src/lib/catalog.ts`)
+
+- **Tool:** GitHub Copilot
+- **What the team did:** Provided the detail-page design and specified modular
+  sections, reuse of the existing variant and difficulty components, existing
+  CSS colors only, and accessibility requirements.
+- **What GitHub Copilot did:** Added a route-backed page, a local catalog data
+  adapter, reusable header, metadata, diagram and action sections, and tests
+  for catalog lookup and variant navigation. The page composes the existing
+  `ElementVariants` and `DifficultyBadge` components.
+- **Review:** The team checked the page in the browser, corrected theme-token
+  use and image sizing, and reviewed keyboard navigation and accessible names.
+
 ## Missing fields in the GraphQL schema (`backend/src/schema.graphql`, `docs/API.md`)
 
 - **Tool:** Claude Code, and Claude for the research.