feat(ui): implement dashboard layout and refactor pages

This commit replaces the previous simple layout with a comprehensive dashboard interface using Nuxt UI Pro components.

- Introduced a new default layout with `UDashboardSidebar` and `UDashboardPanel`.
- Refactored the main page (`index.vue`) by splitting it into `ItemManageStatistics` and `ItemManageTable` components.
- Added new computed stats for monthly growth and total images, and displayed them in new statistic cards.
- Reorganized components from `item/` to a new `itemManage/` directory.
- Added custom primary and secondary theme colors.

app/assets/css/main.css

@@ -1,6 +1,11 @@
 @import "tailwindcss";
 @import "@nuxt/ui";
 
+@theme {
+  --color-primary: #3a4dfb;
+  --color-secondary: #919cfc;
+}
+
 .router-link-exact-active {
   @apply text-blue-600 bg-blue-50;
 }