feat(06-03): create useFirstRunState derived hook

- Derives first-run state from cached useCategories + useTemplate queries
- Returns { isFirstRun, loading } with no additional network calls
- isFirstRun true when categories or template items array is empty
- Loading guard prevents spurious redirects during fetch

src/hooks/useFirstRunState.ts

@@ -0,0 +1,28 @@
+import { useCategories } from "@/hooks/useCategories"
+import { useTemplate } from "@/hooks/useTemplate"
+
+/**
+ * Derives first-run state from cached category and template queries.
+ * No additional network calls are made — data is read from React Query cache.
+ *
+ * isFirstRun is true when:
+ *   - categories.length === 0 (user has not created any categories), OR
+ *   - items.length === 0 (user has no template items)
+ *
+ * IMPORTANT: Always check `loading` before acting on `isFirstRun`.
+ * While queries are in flight, both arrays default to [] which would
+ * cause isFirstRun to be true spuriously.
+ *
+ * Usage:
+ *   const { isFirstRun, loading } = useFirstRunState()
+ *   if (!loading && isFirstRun) { redirect to /setup }
+ */
+export function useFirstRunState() {
+  const { categories, loading: catLoading } = useCategories()
+  const { items, loading: tmplLoading } = useTemplate()
+
+  return {
+    isFirstRun: categories.length === 0 || items.length === 0,
+    loading: catLoading || tmplLoading,
+  }
+}