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/style.css

@@ -0,0 +1,167 @@
+:root {
+  font-family: system-ui, Avenir, Helvetica, Arial, sans-serif;
+  line-height: 1.5;
+  font-weight: 400;
+
+  color-scheme: light dark;
+  color: rgba(255, 255, 255, 0.87);
+  background-color: #242424;
+
+  font-synthesis: none;
+  text-rendering: optimizeLegibility;
+  -webkit-font-smoothing: antialiased;
+  -moz-osx-font-smoothing: grayscale;
+}
+
+a {
+  font-weight: 500;
+  color: #646cff;
+  text-decoration: inherit;
+}
+a:hover {
+  color: #535bf2;
+}
+
+body {
+  margin: 0;
+  display: flex;
+  place-items: center;
+  min-width: 320px;
+  min-height: 100vh;
+}
+
+h1 {
+  font-size: 3.2em;
+  line-height: 1.1;
+}
+
+button {
+  border-radius: 8px;
+  border: 1px solid transparent;
+  padding: 0.6em 1.2em;
+  font-size: 1em;
+  font-weight: 500;
+  font-family: inherit;
+  background-color: #1a1a1a;
+  cursor: pointer;
+  transition: border-color 0.25s;
+}
+button:hover {
+  border-color: #646cff;
+}
+button:focus,
+button:focus-visible {
+  outline: 4px auto -webkit-focus-ring-color;
+}
+
+.card {
+  padding: 2em;
+}
+
+#app {
+  max-width: 1280px;
+  margin: 0 auto;
+  padding: 2rem;
+  text-align: center;
+}
+
+/* Layout */
+.editor-container {
+  display: flex;
+  flex-direction: row;
+  height: 100vh;
+  width: 100%;
+}
+@media (max-width: 768px) {
+  .editor-container {
+    flex-direction: column;
+  }
+}
+.editor, .preview {
+  flex: 1;
+  padding: 1rem;
+  box-sizing: border-box;
+}
+
+/* Editor textarea */
+.editor textarea,
+.editor input,
+.editor .input {
+  border: none;
+  outline: none;
+  background: transparent;
+  line-height: 1.6;
+  font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
+  width: 100%;
+  height: 100%;
+  resize: none;
+}
+
+/* Selection */
+.editor ::selection {
+  background: rgba(200, 200, 200, 0.3);
+}
+
+/* Preview typography */
+.preview p {
+  line-height: 1.6;
+}
+.preview h1 { font-size: 2em; }
+.preview h2 { font-size: 1.75em; }
+.preview h3 { font-size: 1.5em; }
+.preview h4 { font-size: 1.25em; }
+.preview h5 { font-size: 1em; }
+.preview h6 { font-size: 0.875em; }
+.preview blockquote {
+  border-left: 4px solid #ddd;
+  padding-left: 1rem;
+  color: #666;
+}
+
+/* Tables */
+.preview table {
+  border-collapse: collapse;
+  width: 100%;
+}
+.preview th,
+.preview td {
+  border: 1px solid #ccc;
+  padding: 0.4rem;
+}
+
+/* Code block */
+.preview pre,
+.preview code {
+  background: #f5f5f5;
+  border-radius: 4px;
+  padding: 0.8rem;
+  overflow-x: auto;
+}
+
+/* Global */
+html,
+body {
+  margin: 0;
+  padding: 0;
+  height: 100%;
+}
+
+/* CSS variables (to be defined elsewhere) */
+:root {
+  --bg-color: var(--bg-color);
+  --text-color: var(--text-color);
+}
+
+/* Light mode overrides (original) */
+@media (prefers-color-scheme: light) {
+  :root {
+    color: #213547;
+    background-color: #ffffff;
+  }
+  a:hover {
+    color: #747bff;
+  }
+  button {
+    background-color: #f9f9f9;
+  }
+}