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/StageColumn.vue

@@ -0,0 +1,75 @@
+<template>
+  <div class="rounded-lg border border-border bg-panel">
+    <div class="flex items-center justify-between px-3 py-2 border-b border-border">
+      <div class="font-semibold">{{ stage?.title || '未命名阶段' }}</div>
+      <div class="flex items-center gap-2 text-sm">
+        <button class="px-2 py-1 rounded bg-slate-800/60 border border-border hover:bg-slate-800" @click="onAdd">添加任务</button>
+        <button class="px-2 py-1 rounded bg-slate-800/60 border border-border hover:bg-slate-800" @click="onRename">重命名</button>
+        <button class="px-2 py-1 rounded bg-slate-800/60 border border-border hover:bg-slate-800" @click="onMoveCol">移列</button>
+        <button class="px-2 py-1 rounded bg-slate-800/60 border border-border hover:bg-slate-800" @click="onDelete">删除</button>
+      </div>
+    </div>
+    <div class="p-2 flex flex-col gap-2 min-h-[60px]" :data-stage-id="stageId" @dragover.prevent="onDragOver" @dragleave="over=false" @drop.prevent="onDrop" :class="{ 'outline outline-2 outline-accent/60 outline-offset-[-4px] rounded-md': over }">
+      <TaskCard v-for="tid in filteredTasks" :key="tid" :task-id="tid" @dragging="(v) => dragging.value = v" />
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { useBoardStore } from '~/stores/board'
+import TaskCard from './TaskCard.vue'
+
+const store = useBoardStore()
+const props = defineProps<{ stageId: string; query: string; category: string | '' }>()
+const stage = computed(() => store.stageById(props.stageId))
+const tasks = computed(() => stage.value?.tasks || [])
+const filteredTasks = computed(() => {
+  const q = (props.query || '').toLowerCase()
+  const c = props.category
+  return tasks.value.filter((tid) => {
+    const t = store.taskById(tid)
+    if (!t) return false
+    const hitText = !q || t.title.toLowerCase().includes(q) || (t.description || '').toLowerCase().includes(q)
+    const hitCat = !c || t.category === c
+    return hitText && hitCat
+  })
+})
+
+const over = ref(false)
+const dragging = ref(false)
+function onDragOver(e: DragEvent) {
+  e.dataTransfer!.dropEffect = 'move'
+  over.value = true
+}
+function onDrop(e: DragEvent) {
+  over.value = false
+  const taskId = e.dataTransfer?.getData('text/task'); if (!taskId) return
+  const list = (e.currentTarget as HTMLElement).querySelectorAll('[data-task]')
+  const y = e.clientY
+  let index = list.length
+  for (let i = 0; i < list.length; i++) {
+    const rect = (list[i] as HTMLElement).getBoundingClientRect()
+    if (y < rect.top + rect.height / 2) { index = i; break }
+  }
+  store.moveTask(taskId, props.stageId, index)
+}
+function onAdd() { store.addTask(props.stageId) }
+function onRename() {
+  const title = prompt('新的阶段名称？', stage.value?.title || '')
+  if (!title) return
+  store.renameStage(props.stageId, title.trim())
+}
+function onDelete() {
+  if (!confirm('删除该阶段？仅空阶段可删除')) return
+  if (!store.deleteStage(props.stageId)) alert('阶段非空或不存在')
+}
+function onMoveCol() {
+  const cols = store.board.layout.columns
+  if (!cols.length) return alert('暂无列')
+  const ans = prompt(`移动到第几列（1 - ${cols.length}）?`, '1')
+  const target = Math.max(1, Math.min(Number(ans) || 1, cols.length)) - 1
+  cols.forEach((col) => { const i = col.indexOf(props.stageId); if (i !== -1) col.splice(i, 1) })
+  cols[target].push(props.stageId)
+  store.log('stage-move-column', { id: props.stageId, to: target })
+}
+</script>