feat(project): scaffold Vue 3 + Vite markdown editor

Add initial project structure including:
- .gitignore and VSCode settings
- Vite configuration and package.json with Vue 3 dependencies
- Basic HTML entry point and README update
- Core source files: App.vue, main.js, style.css
- Markdown editor component with plugin system and related types
- Sample HelloWorld component, router, and Pinia store
- Assets and SVG icons

This commit establishes the foundation for the Vue 3 application.

src/plugins/index.ts

@@ -0,0 +1,7 @@
+import type { MarkdownPlugin } from './types'
+
+/**
+ * Global plugin registry.
+ * Plugins can implement lifecycle hooks defined in {@link MarkdownPlugin}.
+ */
+export const plugins: MarkdownPlugin[] = []