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/Toolbar.vue

@@ -0,0 +1,106 @@
+<template>
+  <div class="flex flex-wrap items-center gap-2 p-3 border-b border-border bg-panel">
+    <div class="flex items-center gap-2">
+      <h1 class="text-lg font-semibold">{{ appTitle }}</h1>
+      <span class="text-xs text-slate-400">{{ fileStatus }}</span>
+    </div>
+    <div class="flex-1" />
+    <div class="flex items-center gap-2">
+      <label class="px-3 py-1 rounded bg-slate-800/60 border border-border cursor-pointer hover:bg-slate-800">
+        <input type="file" class="hidden" accept="application/json" @change="onOpen" />
+        打开…
+      </label>
+      <button class="px-3 py-1 rounded bg-accent text-slate-900 font-medium hover:brightness-110" @click="onSave">保存/导出</button>
+      <label class="px-3 py-1 rounded bg-slate-800/60 border border-border cursor-pointer hover:bg-slate-800">
+        <input type="file" class="hidden" accept="application/json" @change="onImport" />
+        导入/合并…
+      </label>
+      <button class="px-3 py-1 rounded bg-slate-800/60 border border-border hover:bg-slate-800" @click="onNew">新建</button>
+      <span class="w-px h-6 bg-border mx-1" />
+      <input v-model="actor" placeholder="你的名字（记录历史）" class="px-2 py-1 rounded bg-slate-900 border border-border w-48" />
+      <span class="w-px h-6 bg-border mx-1" />
+      <input v-model="q" placeholder="搜索任务…" class="px-2 py-1 rounded bg-slate-900 border border-border w-56" />
+      <select v-model="cat" class="px-2 py-1 rounded bg-slate-900 border border-border">
+        <option value="">筛选：全部类别</option>
+        <option v-for="c in store.board.categories" :key="c.uuid" :value="c.uuid">{{ c.title }}</option>
+      </select>
+      <span class="w-px h-6 bg-border mx-1" />
+      <button class="px-3 py-1 rounded bg-slate-800/60 border border-border hover:bg-slate-800" @click="onLoadLocal">从本地读取</button>
+      <button class="px-3 py-1 rounded bg-slate-800/60 border border-border hover:bg-slate-800" @click="onSaveLocal">保存到本地</button>
+      <button class="px-3 py-1 rounded bg-slate-800/60 border border-border hover:bg-slate-800" @click="onClearLocal">清空本地</button>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { useBoardStore } from '~/stores/board'
+
+const store = useBoardStore()
+const config = useRuntimeConfig().public
+const appTitle = computed(() => config.appTitle)
+const actor = computed({
+  get: () => store.board.meta?.actor || '',
+  set: (v: string) => store.setActor(v)
+})
+const q = defineModel<string>('query', { required: true })
+const cat = defineModel<string | ''>('category', { required: true })
+
+const fileStatus = computed(() => {
+  const name = store.filename || '(未命名)'
+  const a = store.board.meta?.actor ? ` | 操作人：${store.board.meta.actor}` : ''
+  const star = store.dirty ? ' *' : ''
+  return `${name}${star}${a}`
+})
+
+function onOpen(e: Event) {
+  const input = e.target as HTMLInputElement
+  const file = input.files?.[0]
+  if (!file) return
+  const reader = new FileReader()
+  reader.onload = () => {
+    try {
+      const json = JSON.parse(String(reader.result))
+      store.setBoard(json, file.name)
+      store.log('load-file', { name: file.name, size: file.size })
+    } catch (err: any) {
+      alert('解析 JSON 失败：' + err.message)
+    }
+  }
+  reader.readAsText(file)
+  input.value = ''
+}
+function onSave() { store.downloadCurrent() }
+function onNew() {
+  if (store.dirty && !confirm('当前更改尚未保存，确定新建吗？')) return
+  store.setBoard({ categories: [], stages: [], tasks: [], layout: { columns: [] }, meta: { id: 'open-kanban', version: '1.0.0', createdAt: new Date().toISOString(), modifiedAt: new Date().toISOString(), actor: store.board.meta?.actor, history: [] } }, '')
+  store.log('new-board', {})
+}
+function onImport(e: Event) {
+  const input = e.target as HTMLInputElement
+  const file = input.files?.[0]
+  if (!file) return
+  const reader = new FileReader()
+  reader.onload = () => {
+    try {
+      const data = JSON.parse(String(reader.result))
+      const diff = store.diffBoards(store.board as any, data)
+      // emit event for MergeModal
+      emit('open-merge', data, diff, file.name)
+    } catch (err: any) {
+      alert('解析 JSON 失败：' + err.message)
+    }
+  }
+  reader.readAsText(file)
+  input.value = ''
+}
+function onLoadLocal() {
+  if (!store.loadFromLocal()) alert('本地不存在保存的数据')
+}
+function onSaveLocal() { store.saveToLocal() }
+function onClearLocal() {
+  if (confirm('清空本地保存？')) store.clearLocal()
+}
+
+const emit = defineEmits<{ (e: 'open-merge', imported: any, diff: any, name: string): void }>()
+</script>
+