feat: add item duplication with copy-and-edit workflow

Adds POST /api/items/:id/duplicate endpoint, useDuplicateItem hook, and a
Duplicate button on ItemCard (collection view only) that opens the new item
for editing immediately after creation.

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

src/server/routes/items.ts

@@ -7,6 +7,7 @@ import { parseId } from "../lib/params.ts";
 import {
 	createItem,
 	deleteItem,
+	duplicateItem,
 	getAllItems,
 	getItemById,
 	updateItem,
@@ -52,6 +53,15 @@ app.put(
 	},
 );
 
+app.post("/:id/duplicate", (c) => {
+	const db = c.get("db");
+	const id = parseId(c.req.param("id"));
+	if (!id) return c.json({ error: "Invalid item ID" }, 400);
+	const newItem = duplicateItem(db, id);
+	if (!newItem) return c.json({ error: "Item not found" }, 404);
+	return c.json(newItem, 201);
+});
+
 app.delete("/:id", async (c) => {
 	const db = c.get("db");
 	const id = parseId(c.req.param("id"));