feat(frontend): add Tabs primitive + useHashTab hook

- useHashTab: pure-TS hook reading window.location.hash, falls back to
  defaultId, hashchange listener cleaned up on unmount
- Tabs: <Tabs items activeId onChange> with tab-underline / tab-underline-active
  recipes and count-pill (rounded-pill exception per DESIGN.md)
- index.css: tab-underline*, tab-count-pill*, alert-*, table-compact recipes

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

frontend/src/hooks/useHashTab.ts

@@ -0,0 +1,25 @@
+import { useCallback, useEffect, useState } from 'react';
+
+function readHash(defaultId: string): string {
+  const hash = window.location.hash.slice(1); // strip leading '#'
+  return hash || defaultId;
+}
+
+export function useHashTab(defaultId: string): [string, (id: string) => void] {
+  const [activeId, setActiveId] = useState<string>(() => readHash(defaultId));
+
+  useEffect(() => {
+    function onHashChange() {
+      setActiveId(readHash(defaultId));
+    }
+    window.addEventListener('hashchange', onHashChange);
+    return () => window.removeEventListener('hashchange', onHashChange);
+  }, [defaultId]);
+
+  const navigate = useCallback((id: string) => {
+    window.location.hash = id;
+    setActiveId(id);
+  }, []);
+
+  return [activeId, navigate];
+}