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/MergeModal.vue

@@ -0,0 +1,66 @@
+<template>
+  <div v-if="open" class="fixed inset-0 bg-black/60 flex items-center justify-center p-4">
+    <div class="w-[1000px] max-w-[95vw] max-h-[90vh] rounded-lg border border-border bg-panel flex flex-col">
+      <div class="flex items-center justify-between px-3 py-2 border-b border-border">
+        <h2 class="font-semibold">导入/合并 预览</h2>
+        <button class="px-2 py-1 rounded bg-slate-800/60 border border-border hover:bg-slate-800" @click="close">✕</button>
+      </div>
+      <div class="p-3 overflow-auto space-y-3">
+        <div class="flex items-center gap-3">
+          <div>
+            <label class="text-sm text-slate-400 mr-2">策略</label>
+            <select v-model="policy" class="px-2 py-1 rounded bg-slate-900 border border-border">
+              <option value="prefer-import">冲突优先：导入文件</option>
+              <option value="prefer-current">冲突优先：当前看板</option>
+            </select>
+          </div>
+          <label class="text-sm flex items-center gap-2">
+            <input type="checkbox" v-model="removeMissing" /> 同步删除在导入文件中不存在的任务/阶段
+          </label>
+        </div>
+        <div class="grid grid-cols-3 gap-2">
+          <div class="rounded border border-border bg-slate-900 p-3">
+            <div class="text-sm text-slate-400">任务新增</div>
+            <div class="text-2xl font-bold">{{ diff?.tasks?.added?.length || 0 }}</div>
+          </div>
+          <div class="rounded border border-border bg-slate-900 p-3">
+            <div class="text-sm text-slate-400">任务删除</div>
+            <div class="text-2xl font-bold">{{ diff?.tasks?.removed?.length || 0 }}</div>
+          </div>
+          <div class="rounded border border-border bg-slate-900 p-3">
+            <div class="text-sm text-slate-400">任务修改</div>
+            <div class="text-2xl font-bold">{{ diff?.tasks?.modified?.length || 0 }}</div>
+          </div>
+        </div>
+        <div class="rounded border border-border bg-slate-900 p-3 text-xs whitespace-pre-wrap">
+          <div>文件：{{ name }}</div>
+          <div>Tasks: +{{ diff?.tasks?.added?.length || 0 }} -{{ diff?.tasks?.removed?.length || 0 }} ~{{ diff?.tasks?.modified?.length || 0 }}</div>
+          <div>Stages: +{{ diff?.stages?.added?.length || 0 }} -{{ diff?.stages?.removed?.length || 0 }} ~{{ diff?.stages?.modified?.length || 0 }}</div>
+          <div>Layout changed: {{ diff?.layout?.changed ? 'Yes' : 'No' }}</div>
+        </div>
+      </div>
+      <div class="flex items-center gap-2 px-3 py-2 border-t border-border">
+        <button class="px-3 py-1 rounded bg-accent text-slate-900 font-medium hover:brightness-110" @click="apply">应用合并</button>
+      </div>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { useBoardStore } from '~/stores/board'
+
+const store = useBoardStore()
+const open = defineModel<boolean>('open', { required: true })
+const imported = defineModel<any>('imported', { required: true })
+const diff = defineModel<any>('diff', { required: true })
+const name = defineModel<string>('name', { required: true })
+
+const policy = ref<'prefer-import' | 'prefer-current'>('prefer-import')
+const removeMissing = ref(false)
+function close(){ open.value = false }
+function apply(){
+  store.applyMerge(imported.value, policy.value, removeMissing.value)
+  close()
+}
+</script>
+