feat: add CSV import/export for gear collection

Adds export (GET /api/items/export) and import (POST /api/items/import) routes
backed by a pure csv.service with no external deps, plus useExportItems/useImportItems
hooks and an Import/Export section in the Settings page.

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

src/client/hooks/useItems.ts

@@ -1,6 +1,6 @@
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import type { CreateItem } from "../../shared/types";
-import { apiDelete, apiGet, apiPost, apiPut } from "../lib/api";
+import { apiDelete, apiGet, apiPost, apiPut, apiUpload } from "../lib/api";
 
 interface Item {
 	id: number;
@@ -96,3 +96,27 @@ export function useDuplicateItem() {
 		},
 	});
 }
+
+export function useExportItems() {
+	return function exportItems() {
+		window.location.href = "/api/items/export";
+	};
+}
+
+interface ImportResult {
+	imported: number;
+	createdCategories: string[];
+	errors: string[];
+}
+
+export function useImportItems() {
+	const queryClient = useQueryClient();
+	return useMutation({
+		mutationFn: (file: File) =>
+			apiUpload<ImportResult>("/api/items/import", file),
+		onSuccess: () => {
+			queryClient.invalidateQueries({ queryKey: ["items"] });
+			queryClient.invalidateQueries({ queryKey: ["totals"] });
+		},
+	});
+}