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.

types/schema.ts

@@ -0,0 +1,54 @@
+export interface Category {
+  uuid: string
+  title: string
+  color?: string
+}
+
+export interface Step {
+  details: string
+  done: boolean
+}
+
+export interface Task {
+  uuid: string
+  title: string
+  description?: string
+  category?: string | null
+  steps?: Step[]
+}
+
+export interface Stage {
+  uuid: string
+  title: string
+  tasks: string[]
+}
+
+export interface Layout {
+  columns: string[][]
+}
+
+export interface HistoryEntry {
+  id: string
+  ts: string
+  actor?: string
+  type: string
+  payload?: any
+}
+
+export interface Meta {
+  id: 'open-kanban'
+  version: string
+  createdAt: string
+  modifiedAt: string
+  actor?: string
+  history: HistoryEntry[]
+}
+
+export interface Board {
+  categories: Category[]
+  stages: Stage[]
+  tasks: Task[]
+  layout: Layout
+  meta?: Meta
+}
+