feat: add day/week/month period toggle for dashboard line charts

Add a period selector (day, week, month) to the reading activity and
books added charts. The API now accepts a ?period= query param and
returns gap-filled data using generate_series so all time slots appear
even with zero values. Labels are locale-aware (short month, weekday).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/backoffice/lib/api.ts

@@ -584,8 +584,9 @@ export type StatsResponse = {
   metadata: MetadataStats;
 };
 
-export async function fetchStats() {
-  return apiFetch<StatsResponse>("/stats", { next: { revalidate: 30 } });
+export async function fetchStats(period?: "day" | "week" | "month") {
+  const params = period && period !== "month" ? `?period=${period}` : "";
+  return apiFetch<StatsResponse>(`/stats${params}`, { next: { revalidate: 30 } });
 }
 
 // ---------------------------------------------------------------------------