feat(ui): overhaul interface with Nuxt UI

Integrate the Nuxt UI component library and completely revamp the application's user interface to improve usability, aesthetics, and maintainability.

- Replace all custom components and native browser dialogs (`alert`, `prompt`, `confirm`) with Nuxt UI components like `UCard`, `UButton`, `UModal`, and `UNotifications`.
- Refactor the main page by extracting the sidebar into dedicated panel components: `CategoryPanel`, `BoardSummaryPanel`, and `HistoryPanel`.
- Redesign all major components (Toolbar, Board, Stage, Task) for a cleaner layout and improved information hierarchy.
- Implement user-friendly modals for all creation, editing, and deletion flows, providing a more consistent user experience.
- Add toast notifications to provide immediate feedback for user actions.

nuxt.config.ts

@@ -1,6 +1,6 @@
 // Nuxt 4 configuration
 export default defineNuxtConfig({
-  modules: ['@pinia/nuxt', '@nuxtjs/tailwindcss'],
+  modules: ['@nuxt/ui', '@pinia/nuxt', '@nuxtjs/tailwindcss'],
   css: ['~/assets/css/tailwind.css'],
   typescript: {
     strict: true
@@ -10,6 +10,11 @@ export default defineNuxtConfig({
       title: 'Kanban'
     }
   },
+  ui: {
+    global: true,
+    primary: 'sky',
+    gray: 'slate'
+  },
   tailwindcss: {
     viewer: false
   },
@@ -22,4 +27,3 @@ export default defineNuxtConfig({
     }
   }
 });
-