feat(web): add M0 live preview editor prototype

- CodeMirror 6 editor with Obsidian/Typora-style live preview: block
  markup revealed per line, inline markup revealed per element
- Fenced code highlighting via Shiki (github-dark, JS regex engine) in a
  Web Worker, languages loaded on demand
- Lezer syntax tree to HTML renderer for share pages, checked against
  the CommonMark 0.31.2 spec (640/652; 12 known parser deviations)
- Use markdownLanguage directly to avoid bundling lang-html/css/js

web/src/highlight/protocol.ts

@@ -0,0 +1,16 @@
+// 主线程与高亮 Worker 之间的消息格式
+
+export interface HighlightRequest {
+  id: number
+  lang: string
+  code: string
+}
+
+// 扁平数组：[offset, length, color, fontStyle, offset, length, ...]，避免大量小对象跨线程拷贝
+export type TokenRun = (number | string)[]
+
+export interface HighlightResponse {
+  id: number
+  // 语言不受支持时为 null，调用方按纯文本显示
+  tokens: TokenRun | null
+}