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/dialogs/StageModal.vue

@@ -0,0 +1,76 @@
+<template>
+  <UModal v-model="open">
+    <UCard>
+      <template #header>
+        <div class="flex items-center gap-2">
+          <UIcon name="i-heroicons-plus-circle-20-solid" class="h-5 w-5 text-sky-400" />
+          <span class="font-semibold">添加阶段</span>
+        </div>
+      </template>
+
+      <form class="space-y-4" @submit.prevent="handleSubmit">
+        <UFormGroup label="阶段名称" name="title">
+          <UInput v-model="form.title" placeholder="例如：待办、进行中、已完成…" autofocus />
+        </UFormGroup>
+
+        <UFormGroup v-if="columnOptions.length" label="放置到列" name="column">
+          <USelectMenu
+            v-model="form.column"
+            :options="columnOptions"
+            value-attribute="value"
+            option-attribute="label"
+          />
+        </UFormGroup>
+        <p v-else class="text-xs text-slate-400">
+          尚未创建列，新增阶段时系统会自动建立第一列。
+        </p>
+
+        <div class="flex justify-end gap-2">
+          <UButton color="neutral" variant="ghost" @click="open = false">取消</UButton>
+          <UButton type="submit" color="primary">创建阶段</UButton>
+        </div>
+      </form>
+    </UCard>
+  </UModal>
+</template>
+
+<script setup lang="ts">
+import { useBoardStore } from '~/stores/board'
+
+const store = useBoardStore()
+const toast = useToast()
+
+const props = defineProps<{ defaultColumn?: number }>()
+const open = defineModel<boolean>('open', { required: true })
+const emit = defineEmits<{ (e: 'submit', payload: { title: string; column: number }): void }>()
+
+const form = reactive({
+  title: '',
+  column: 0
+})
+
+const columnOptions = computed(() =>
+  (store.board.layout?.columns || []).map((_, index) => ({
+    label: `列 ${index + 1}`,
+    value: index
+  }))
+)
+
+watch(
+  () => open.value,
+  (value) => {
+    if (!value) return
+    form.title = ''
+    const defaultColumn = props.defaultColumn ?? 0
+    form.column = columnOptions.value[defaultColumn] ? defaultColumn : 0
+  }
+)
+
+function handleSubmit() {
+  if (!form.title.trim()) {
+    toast.add({ color: 'rose', title: '请输入阶段名称' })
+    return
+  }
+  emit('submit', { title: form.title.trim(), column: form.column })
+}
+</script>