refactor(app): rewrite application with Nuxt 4, Pinia, and TailwindCSS

This commit replaces the original vanilla JavaScript implementation with a modern frontend stack. The entire application has been rewritten to improve maintainability, developer experience, and leverage modern
web technologies.

Key changes include:
- Replaced plain HTML, CSS, and JS with a Nuxt 4 project structure.
- Migrated state management from a global state object to a Pinia store for better organization and reactivity.
- Rebuilt the UI with Vue 3 components and styled with TailwindCSS.
- Changed the primary persistence mechanism from manual file I/O to automatic LocalStorage saving, while retaining import/export functionality.
- All core features, including drag-and-drop, task management, and the import/merge diffing logic, have been ported to the new architecture.

components/Board.vue

@@ -0,0 +1,44 @@
+<template>
+  <div class="p-3 overflow-auto grid gap-3" :style="gridStyle">
+    <div v-for="(col, colIndex) in columns" :key="colIndex" class="flex flex-col gap-3 min-w-[320px]">
+      <StageColumn v-for="sid in col" :key="sid" :stage-id="sid" :query="query" :category="category" />
+    </div>
+  </div>
+  <div v-if="!columns.length || !columns[0]?.length" class="text-center text-slate-400 py-10">
+    暂无阶段。点击左侧“添加阶段”创建。
+  </div>
+  <div class="px-3 pb-3">
+    <button class="px-3 py-1 rounded bg-slate-800/60 border border-border hover:bg-slate-800" @click="addColumn">添加列</button>
+  </div>
+  <div class="px-3 pb-3">
+    <button class="px-3 py-1 rounded bg-slate-800/60 border border-border hover:bg-slate-800" @click="addStage">添加阶段</button>
+  </div>
+  
+</template>
+
+<script setup lang="ts">
+import { useBoardStore } from '~/stores/board'
+import StageColumn from './StageColumn.vue'
+
+const store = useBoardStore()
+const props = defineProps<{ query: string; category: string | '' }>()
+const columns = computed(() => store.board.layout?.columns || [])
+
+const gridStyle = computed(() => ({
+  gridAutoFlow: 'column',
+  gridAutoRows: '1fr'
+}))
+
+function addColumn() {
+  if (!store.board.layout.columns) store.board.layout.columns = []
+  store.board.layout.columns.push([])
+  store.log('column-add', { count: store.board.layout.columns.length })
+}
+function addStage() {
+  const title = prompt('阶段名称？')
+  if (!title) return
+  const colIndex = store.board.layout.columns.length ? 0 : 0
+  store.addStage(title.trim(), colIndex)
+}
+</script>
+