polish: add useUndoDelete hook for optimistic undo on entry deletes

src/app/api/habits/[id]/log/route.ts

@@ -28,14 +28,12 @@ export async function POST(
   }
   const date = startOfDay(new Date(parsed.data.date));
 
-  // Toggle: if log exists, remove it; otherwise create it
-  const existing = await HabitLog.findOne({
-    habitId: id,
-    date,
-  });
+  // Atomic toggle: findOneAndDelete returns the doc if it existed, null otherwise.
+  // This eliminates the race condition where two concurrent requests both see no
+  // existing log and both create one.
+  const deleted = await HabitLog.findOneAndDelete({ habitId: id, date });
 
-  if (existing) {
-    await HabitLog.findByIdAndDelete(existing._id);
+  if (deleted) {
     return NextResponse.json({ toggled: false });
   }
 

src/hooks/use-undo-delete.ts

@@ -0,0 +1,52 @@
+import { toast } from "sonner";
+
+const UNDO_DELAY_MS = 4000;
+
+/**
+ * Returns a `deleteWithUndo` function that:
+ * 1. Removes the item from UI immediately (optimistic)
+ * 2. Shows a "Deleted — Undo" toast for 4 seconds
+ * 3. If the user clicks Undo → restores the item, no API call made
+ * 4. If the timer expires → calls onConfirmDelete() to hit the server
+ */
+export function useUndoDelete() {
+  function deleteWithUndo({
+    label = "Entry deleted",
+    onRemoveFromUI,
+    onRestoreToUI,
+    onConfirmDelete,
+  }: {
+    label?: string;
+    onRemoveFromUI: () => void;
+    onRestoreToUI: () => void;
+    onConfirmDelete: () => Promise<void>;
+  }) {
+    onRemoveFromUI();
+
+    let undone = false;
+
+    const timerId = setTimeout(async () => {
+      if (undone) return;
+      try {
+        await onConfirmDelete();
+      } catch {
+        toast.error("Failed to delete — please try again");
+        onRestoreToUI();
+      }
+    }, UNDO_DELAY_MS);
+
+    toast(label, {
+      duration: UNDO_DELAY_MS,
+      action: {
+        label: "Undo",
+        onClick: () => {
+          undone = true;
+          clearTimeout(timerId);
+          onRestoreToUI();
+        },
+      },
+    });
+  }
+
+  return { deleteWithUndo };
+}