docs: publish the UI standards in docs/contributing

The binding UI rulebook (card anatomy, spacing scale, text colour,
tokens, primitive mapping, enforcement) lived outside the repository,
so a contributor could follow a code comment to a file they could not
open. It now lives at docs/contributing/ui-standards.md, every comment
that cited it points there, and the docs index and ui-guidelines.md
say which document wins when they disagree.

src/components/ui/query-error-row.tsx

@@ -17,7 +17,7 @@ import { cn } from "@/lib/utils";
  * geometry and pushes everything the user came for below the fold — so this
  * shape stays, promoted to a primitive instead of being copied a third time.
  *
- * Pick by surface (UI-STANDARDS §6):
+ * Pick by surface (ui-standards.md §6):
  *  - primary / list surface → `QueryErrorCard`
  *  - dense metric strip or tile where the tall card would dominate →
  *    `QueryErrorRow`