feat(01-04): add onboarding wizard with settings API and persisted state

- Settings API: GET/PUT /api/settings/:key with SQLite persistence
- useSettings hook with TanStack Query for settings CRUD
- OnboardingWizard: 3-step modal overlay (welcome, create category, add item)
- Root layout checks onboarding completion flag before rendering wizard
- Skip option available at every step, all paths persist completion to DB

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/client/hooks/useSettings.ts

@@ -0,0 +1,37 @@
+import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
+import { apiGet, apiPut } from "../lib/api";
+
+interface Setting {
+  key: string;
+  value: string;
+}
+
+export function useSetting(key: string) {
+  return useQuery({
+    queryKey: ["settings", key],
+    queryFn: async () => {
+      try {
+        const result = await apiGet<Setting>(`/api/settings/${key}`);
+        return result.value;
+      } catch (err: any) {
+        if (err?.status === 404) return null;
+        throw err;
+      }
+    },
+  });
+}
+
+export function useUpdateSetting() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: ({ key, value }: { key: string; value: string }) =>
+      apiPut<Setting>(`/api/settings/${key}`, { value }),
+    onSuccess: (_data, variables) => {
+      queryClient.invalidateQueries({ queryKey: ["settings", variables.key] });
+    },
+  });
+}
+
+export function useOnboardingComplete() {
+  return useSetting("onboardingComplete");
+}