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.

tailwind.config.ts

@@ -0,0 +1,24 @@
+import type { Config } from 'tailwindcss'
+
+export default {
+  content: [
+    './components/**/*.{vue,js,ts}',
+    './layouts/**/*.vue',
+    './pages/**/*.vue',
+    './plugins/**/*.{js,ts}',
+    './app.vue'
+  ],
+  theme: {
+    extend: {
+      colors: {
+        panel: '#111827',
+        panel2: '#0b1220',
+        border: '#1f2937',
+        card: '#0f172a',
+        accent: '#38bdf8'
+      }
+    }
+  },
+  plugins: []
+} satisfies Config
+