Add Preferences section to the admin Settings screen

Adds a typed settings client (fetchSettings / updateSettings) and a
Preferences section below Children that edits all ten household
settings. Save patches only the changed fields, says "No changes to
save." without a request when nothing changed, shows the 403 refusal
or the API's 422 detail, and shares the one-write-at-a-time lock with
the Children section.

Maestro task: cd3e0209-4575-4acd-bd87-d1595a4451de

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

admin/src/api/settings.ts

@@ -0,0 +1,39 @@
+/**
+ * Typed client for the admin household-settings routes
+ * (/api/v1/admin/settings).
+ */
+import { apiFetch } from "./client";
+import { readJson } from "./definitions";
+
+export interface HouseholdSettings {
+  horizon_days: number;
+  /** Local wall-clock "HH:MM", as are the other *_time fields. */
+  day_rollover_time: string;
+  notify_target: string;
+  morning_summary_time: string;
+  afternoon_reminder_time: string;
+  end_of_day_report_time: string;
+  morning_summary_enabled: boolean;
+  afternoon_reminder_enabled: boolean;
+  end_of_day_report_enabled: boolean;
+  celebration_enabled: boolean;
+}
+
+/** Only the supplied fields are updated; the API answers with all of them. */
+export type SettingsChanges = Partial<HouseholdSettings>;
+
+export const SETTINGS_PATH = "/api/v1/admin/settings";
+
+export async function fetchSettings(): Promise<HouseholdSettings> {
+  return readJson<HouseholdSettings>(await apiFetch(SETTINGS_PATH));
+}
+
+export async function updateSettings(changes: SettingsChanges): Promise<HouseholdSettings> {
+  return readJson<HouseholdSettings>(
+    await apiFetch(SETTINGS_PATH, {
+      method: "PATCH",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify(changes),
+    }),
+  );
+}