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/app/components/PeriodToggle.tsx

@@ -0,0 +1,47 @@
+"use client";
+
+import { useRouter, useSearchParams } from "next/navigation";
+
+type Period = "day" | "week" | "month";
+
+export function PeriodToggle({
+  labels,
+}: {
+  labels: { day: string; week: string; month: string };
+}) {
+  const router = useRouter();
+  const searchParams = useSearchParams();
+  const raw = searchParams.get("period");
+  const current: Period = raw === "day" ? "day" : raw === "week" ? "week" : "month";
+
+  function setPeriod(period: Period) {
+    const params = new URLSearchParams(searchParams.toString());
+    if (period === "month") {
+      params.delete("period");
+    } else {
+      params.set("period", period);
+    }
+    const qs = params.toString();
+    router.push(qs ? `?${qs}` : "/", { scroll: false });
+  }
+
+  const options: Period[] = ["day", "week", "month"];
+
+  return (
+    <div className="flex gap-1 bg-muted rounded-lg p-0.5">
+      {options.map((p) => (
+        <button
+          key={p}
+          onClick={() => setPeriod(p)}
+          className={`px-2.5 py-1 text-xs font-medium rounded-md transition-colors ${
+            current === p
+              ? "bg-card text-foreground shadow-sm"
+              : "text-muted-foreground hover:text-foreground"
+          }`}
+        >
+          {labels[p]}
+        </button>
+      ))}
+    </div>
+  );
+}