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/assets/vue.svg

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+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" aria-hidden="true" role="img" class="iconify iconify--logos" width="37.07" height="36" preserveAspectRatio="xMidYMid meet" viewBox="0 0 256 198"><path fill="#41B883" d="M204.8 0H256L128 220.8L0 0h97.92L128 51.2L157.44 0h47.36Z"></path><path fill="#41B883" d="m0 0l128 220.8L256 0h-51.2L128 132.48L50.56 0H0Z"></path><path fill="#35495E" d="M50.56 0L128 133.12L204.8 0h-47.36L128 51.2L97.92 0H50.56Z"></path></svg>