feat(items): implement initial item management system

This commit introduces the foundational structure and core features for the item management application.

Key features implemented:
- **Item Listing:** A main page displaying all items in a searchable UTable.
- **Item Creation:** An "Add Item" modal with a UForm for input and validation using Zod.
- **State Management:** A `useItemsStore` composable built with `@vueuse/core`'s `useLocalStorage` to persist item data
in the browser.
- **UI Framework:** Integrated Nuxt UI for components like tables, modals, forms, and the overall layout.
- **Application Shell:** Established a default layout with a header, navigation menu, and placeholders for Export and
History pages.
- **Project Setup:** Configured pnpm workspace, Tailwind CSS, and VSCode editor settings for an improved development
experience.

app/composables/items.ts

@@ -0,0 +1,61 @@
+import { useLocalStorage, createSharedComposable } from "@vueuse/core";
+
+// 类型定义
+export type Item = {
+  id: number;
+  name: string;
+  imageUrl?: string | null;
+  description?: string | null;
+  tags?: string[] | null;
+  createdAt: Date;
+  updatedAt: Date;
+};
+
+const _useItems = () => {
+  const items = useLocalStorage<Item[]>("item-database", [], {
+    serializer: {
+      read: (v) => {
+        const parsed = JSON.parse(v);
+        return parsed.map((item: any) => ({
+          ...item,
+          createdAt: new Date(item.createdAt),
+          updatedAt: new Date(item.updatedAt),
+        }));
+      },
+      write: (v) => JSON.stringify(v),
+    },
+  });
+
+  const stats = {
+    totalItems: computed(() => items.value.length),
+    addedThisMonth: computed(() => {
+      const now = new Date();
+      const thisMonth = now.getMonth();
+      const thisYear = now.getFullYear();
+
+      let count = 0;
+      for (const item of items.value) {
+        const date = item.createdAt;
+        if (date.getMonth() === thisMonth && date.getFullYear() === thisYear) {
+          count++;
+        }
+      }
+      return count;
+    }),
+    latestId: computed(
+      () => (items.value[items.value.length - 1]?.id ?? 0) + 1
+    ),
+  };
+
+  const isNameAvailable = (name: string) =>
+    !items.value.some((item) => item.name === name);
+
+  const addItem = (item: Item) => items.value.push(item);
+
+  const removeItem = (id: number) =>
+    (items.value = items.value.filter((i) => i.id !== id));
+
+  return { items, stats, isNameAvailable, addItem, removeItem };
+};
+
+export const useItemsStore = createSharedComposable(_useItems);