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>

tests/routes/items.test.ts

@@ -118,4 +118,30 @@ describe("Item Routes", () => {
 		const res = await app.request("/api/items/9999");
 		expect(res.status).toBe(404);
 	});
+
+	it("POST /api/items/:id/duplicate returns 201 with the copy", async () => {
+		const createRes = await app.request("/api/items", {
+			method: "POST",
+			headers: { "Content-Type": "application/json" },
+			body: JSON.stringify({ name: "Tent", categoryId: 1, weightGrams: 1200 }),
+		});
+		const created = await createRes.json();
+
+		const res = await app.request(`/api/items/${created.id}/duplicate`, {
+			method: "POST",
+		});
+
+		expect(res.status).toBe(201);
+		const body = await res.json();
+		expect(body.name).toBe("Tent (copy)");
+		expect(body.weightGrams).toBe(1200);
+		expect(body.id).not.toBe(created.id);
+	});
+
+	it("POST /api/items/999/duplicate returns 404", async () => {
+		const res = await app.request("/api/items/999/duplicate", {
+			method: "POST",
+		});
+		expect(res.status).toBe(404);
+	});
 });