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/components/Tabs.tsx

@@ -0,0 +1,43 @@
+interface TabItem {
+  id: string;
+  label: string;
+  count?: number;
+}
+
+interface TabsProps {
+  items: TabItem[];
+  activeId: string;
+  onChange: (id: string) => void;
+}
+
+export function Tabs({ items, activeId, onChange }: TabsProps): JSX.Element {
+  return (
+    <div
+      role="tablist"
+      className="flex items-end border-b border-hairline gap-xs"
+    >
+      {items.map((item) => {
+        const isActive = item.id === activeId;
+        return (
+          <button
+            key={item.id}
+            role="tab"
+            aria-selected={isActive}
+            type="button"
+            onClick={() => onChange(item.id)}
+            className={`tab-underline${isActive ? ' tab-underline-active' : ''} pb-xs`}
+          >
+            {item.label}
+            {item.count !== undefined ? (
+              <span
+                className={`ml-xxs tab-count-pill${isActive ? ' tab-count-pill-active' : ''}`}
+              >
+                {item.count}
+              </span>
+            ) : null}
+          </button>
+        );
+      })}
+    </div>
+  );
+}