feat(timeline): duplicate a day's events onto another day

POST /api/events/duplicate-day — copies all events from sourceDate to
targetDate, shifting timestamps by the day offset while preserving
time-of-day, duration, notes, and metadata. Ownership-gated per family.

In the timeline, each day header gains a copy icon button. Clicking it
expands an inline date picker; confirming triggers the API and
invalidates the events query.

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

src/app/(app)/timeline/page.tsx

@@ -8,7 +8,7 @@ import { EventIcon } from "@/components/event-icon";
 import { useBaby } from "@/contexts/baby-context";
 import { format, isToday, isYesterday } from "date-fns";
 import { fr } from "date-fns/locale";
-import { Trash2, StickyNote, Pencil, CheckSquare, Square, XCircle, ImageIcon } from "lucide-react";
+import { Trash2, StickyNote, Pencil, CheckSquare, Square, XCircle, ImageIcon, Copy } from "lucide-react";
 import Image from "next/image";
 import { EventModal } from "@/components/event-modal";
 
@@ -83,6 +83,10 @@ export default function TimelinePage() {
   const [page, setPage] = useState(0);
   const [selectMode, setSelectMode] = useState(false);
   const [selected, setSelected] = useState<Set<string>>(new Set());
+  const [duplicating, setDuplicating] = useState<string | null>(null); // sourceDate being duplicated
+  const [dupTarget, setDupTarget] = useState(""); // target date string
+  const [dupLoading, setDupLoading] = useState(false);
+  const [dupError, setDupError] = useState("");
 
   const { data: eventsData, isLoading } = useQuery({
     queryKey: ["events", selectedBaby?.id, filterType, page],
@@ -106,6 +110,27 @@ export default function TimelinePage() {
     qc.invalidateQueries({ queryKey: ["events"] });
   }
 
+  async function duplicateDay(sourceDate: string, targetDate: string) {
+    if (!selectedBaby?.id) return;
+    setDupLoading(true);
+    setDupError("");
+    const res = await fetch("/api/events/duplicate-day", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ babyId: selectedBaby.id, sourceDate, targetDate }),
+    });
+    setDupLoading(false);
+    if (!res.ok) {
+      const data = await res.json();
+      setDupError(data.error ?? "Erreur");
+      return;
+    }
+    setDuplicating(null);
+    setDupTarget("");
+    setDupError("");
+    qc.invalidateQueries({ queryKey: ["events"] });
+  }
+
   async function deleteSelected() {
     if (selected.size === 0) return;
     await Promise.all([...selected].map((id) => fetch(`/api/events/${id}`, { method: "DELETE" })));
@@ -241,8 +266,48 @@ export default function TimelinePage() {
               <span className="text-xs font-semibold text-slate-500 dark:text-slate-400 uppercase tracking-wide capitalize">{dayLabel(day)}</span>
               <div className="flex-1 h-px bg-slate-200 dark:bg-slate-700" />
               <span className="text-xs text-slate-400 dark:text-slate-500">{grouped[day].length}</span>
+              {!selectMode && (
+                <button
+                  onClick={() => { setDuplicating(day); setDupTarget(""); setDupError(""); }}
+                  title="Dupliquer ce jour"
+                  className="p-1 rounded-lg text-slate-400 hover:text-indigo-600 dark:hover:text-indigo-400 hover:bg-slate-100 dark:hover:bg-slate-800 transition"
+                >
+                  <Copy className="w-3.5 h-3.5" />
+                </button>
+              )}
             </div>
 
+            {duplicating === day && (
+              <div className="mb-3 px-4 py-3 bg-indigo-50 dark:bg-indigo-950/40 border border-indigo-200 dark:border-indigo-800 rounded-xl">
+                <p className="text-xs font-medium text-indigo-700 dark:text-indigo-300 mb-2">
+                  Copier les {grouped[day].length} événements vers…
+                </p>
+                <div className="flex items-center gap-2">
+                  <input
+                    type="date"
+                    value={dupTarget}
+                    onChange={(e) => { setDupTarget(e.target.value); setDupError(""); }}
+                    min={format(new Date(), "yyyy-MM-dd")}
+                    className="flex-1 px-3 py-1.5 text-sm rounded-lg border border-indigo-200 dark:border-indigo-700 bg-white dark:bg-slate-800 text-slate-900 dark:text-slate-100 focus:outline-none focus:ring-2 focus:ring-indigo-400"
+                  />
+                  <button
+                    disabled={!dupTarget || dupLoading}
+                    onClick={() => duplicateDay(day, dupTarget)}
+                    className="px-3 py-1.5 text-xs font-medium bg-indigo-600 hover:bg-indigo-700 disabled:bg-indigo-300 dark:disabled:bg-indigo-900 text-white rounded-lg transition"
+                  >
+                    {dupLoading ? "…" : "Copier"}
+                  </button>
+                  <button
+                    onClick={() => { setDuplicating(null); setDupError(""); }}
+                    className="px-3 py-1.5 text-xs text-slate-500 dark:text-slate-400 border border-slate-200 dark:border-slate-600 rounded-lg bg-white dark:bg-slate-800 hover:text-slate-700 transition"
+                  >
+                    Annuler
+                  </button>
+                </div>
+                {dupError && <p className="mt-1.5 text-xs text-red-500">{dupError}</p>}
+              </div>
+            )}
+
             <div className="bg-white dark:bg-slate-800 border border-slate-200 dark:border-slate-700 rounded-xl overflow-hidden divide-y divide-slate-100 dark:divide-slate-700">
               {grouped[day].map((ev) => {
                 const cfg = EVENT_CONFIG[ev.type];