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>
+  );
+}

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];
+}

frontend/src/styles/index.css

@@ -147,4 +147,48 @@
   .tag-mitre {
     @apply inline-flex items-center bg-cloud text-ink border border-hairline rounded-none px-2 py-[2px] font-mono text-[12px] leading-[1.33];
   }
+
+  /* ─── Sub-page tabs (L1) ─────────────────────────────────────────────── */
+  .tab-underline {
+    @apply text-graphite caption-bold cursor-pointer border-b-2 border-transparent hover:text-ink px-xs;
+  }
+  .tab-underline-active {
+    @apply text-primary border-primary;
+  }
+  /* Count pill — rounded-pill is the allowed exception per DESIGN.md */
+  .tab-count-pill {
+    @apply inline-flex items-center rounded-pill bg-cloud text-graphite text-[11px] px-xs py-0 font-mono;
+  }
+  .tab-count-pill-active {
+    @apply bg-primary-soft text-primary;
+  }
+
+  /* ─── Inline alert banners (L2) ─────────────────────────────────────── */
+  .alert-error {
+    @apply bg-paper border border-hairline border-l-4 border-l-bloom-deep rounded-none px-md py-sm flex items-start gap-sm;
+  }
+  .alert-warn {
+    @apply bg-paper border border-hairline border-l-4 border-l-warn rounded-none px-md py-sm flex items-start gap-sm;
+  }
+  .alert-success {
+    @apply bg-paper border border-hairline border-l-4 border-l-success rounded-none px-md py-sm flex items-start gap-sm;
+  }
+  .alert-info {
+    @apply bg-paper border border-hairline border-l-4 border-l-primary rounded-none px-md py-sm flex items-start gap-sm;
+  }
+
+  /* ─── Compact table density (L4) — 32px row height, global ──────────── */
+  .table-compact thead tr {
+    @apply text-[11px] uppercase tracking-[0.5px] text-graphite;
+  }
+  .table-compact th {
+    @apply px-md py-xxs;
+  }
+  .table-compact td {
+    @apply px-md py-xxs caption-md;
+  }
+  .table-compact tbody tr {
+    @apply border-b border-hairline last:border-0;
+    min-height: 32px;
+  }
 }