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.

components/panels/BoardSummaryPanel.vue

@@ -0,0 +1,73 @@
+<template>
+  <UCard>
+    <template #header>
+      <div class="flex items-center justify-between">
+        <div class="flex items-center gap-2">
+          <UIcon name="i-heroicons-chart-bar-20-solid" class="h-5 w-5 text-emerald-400" />
+          <span class="font-semibold">看板概览</span>
+        </div>
+        <UBadge
+          v-if="lastModified"
+          :label="`更新于：${lastModified}`"
+          color="neutral"
+          variant="soft"
+          class="text-xs"
+        />
+      </div>
+    </template>
+
+    <div class="grid grid-cols-3 gap-3 text-sm">
+      <UCard variant="soft" class="border border-emerald-500/10 bg-emerald-500/5 text-emerald-200">
+        <div class="space-y-1">
+          <p class="text-xs uppercase tracking-wide opacity-80">任务</p>
+          <p class="text-2xl font-semibold">{{ tasksCount }}</p>
+        </div>
+      </UCard>
+      <UCard variant="soft" class="border border-sky-500/10 bg-sky-500/5 text-sky-200">
+        <div class="space-y-1">
+          <p class="text-xs uppercase tracking-wide opacity-80">阶段</p>
+          <p class="text-2xl font-semibold">{{ stagesCount }}</p>
+        </div>
+      </UCard>
+      <UCard variant="soft" class="border border-violet-500/10 bg-violet-500/5 text-violet-200">
+        <div class="space-y-1">
+          <p class="text-xs uppercase tracking-wide opacity-80">列</p>
+          <p class="text-2xl font-semibold">{{ columnsCount }}</p>
+        </div>
+      </UCard>
+    </div>
+
+    <div class="mt-4 space-y-2 text-xs text-slate-400">
+      <div class="flex items-center gap-2">
+        <UIcon name="i-heroicons-user-circle-20-solid" class="h-4 w-4 text-slate-500" />
+        <span>当前操作人：{{ actor || '未设置' }}</span>
+      </div>
+      <div class="flex items-center gap-2">
+        <UIcon name="i-heroicons-document-duplicate-20-solid" class="h-4 w-4 text-slate-500" />
+        <span>文件来源：{{ filename }}</span>
+      </div>
+    </div>
+  </UCard>
+</template>
+
+<script setup lang="ts">
+import { useBoardStore } from '~/stores/board'
+
+const store = useBoardStore()
+
+const columnsCount = computed(() => store.board.layout?.columns?.length || 0)
+const stagesCount = computed(() => store.board.stages.length)
+const tasksCount = computed(() => store.board.tasks.length)
+const actor = computed(() => store.board.meta?.actor || '')
+const filename = computed(() => store.filename || '未命名')
+
+const lastModified = computed(() => {
+  const ts = store.board.meta?.modifiedAt
+  if (!ts) return ''
+  try {
+    return new Intl.DateTimeFormat('zh-CN', { dateStyle: 'short', timeStyle: 'short' }).format(new Date(ts))
+  } catch {
+    return ts
+  }
+})
+</script>