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.

nuxt.config.ts

@@ -0,0 +1,25 @@
+// Nuxt 4 configuration
+export default defineNuxtConfig({
+  modules: ['@pinia/nuxt', '@nuxtjs/tailwindcss'],
+  css: ['~/assets/css/tailwind.css'],
+  typescript: {
+    strict: true
+  },
+  app: {
+    head: {
+      title: 'Kanban'
+    }
+  },
+  tailwindcss: {
+    viewer: false
+  },
+  runtimeConfig: {
+    public: {
+      appTitle: 'Kanban',
+      localStorageKey: 'open-kanban-board',
+      defaultFilename: 'kanban.json',
+      autoLoadFile: '' // e.g. '/sample-board.json'
+    }
+  }
+});
+