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.

app.vue

@@ -1,11 +1,18 @@
 <template>
-  <div class="min-h-full bg-slate-950 text-slate-100">
-    <NuxtPage />
-  </div>
+  <UApp>
+    <NuxtLayout>
+      <NuxtPage />
+    </NuxtLayout>
+    <UNotifications />
+  </UApp>
 </template>
 
 <script setup lang="ts">
-const config = useRuntimeConfig().public
-useHead({ title: config.appTitle })
+const config = useRuntimeConfig().public;
+useHead({
+  titleTemplate: (title) => {
+    if (!title || title === config.appTitle) return config.appTitle;
+    return `${title} · ${config.appTitle}`;
+  },
+});
 </script>
-