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/worker.ts

@@ -0,0 +1,46 @@
+import { createHighlighterCore, type HighlighterCore } from 'shiki/core'
+import { createJavaScriptRegexEngine } from 'shiki/engine/javascript'
+import { bundledLanguages } from 'shiki/langs'
+import githubDark from 'shiki/themes/github-dark.mjs'
+import type { HighlightRequest, HighlightResponse, TokenRun } from './protocol'
+
+const THEME = 'github-dark'
+
+const highlighterReady: Promise<HighlighterCore> = createHighlighterCore({
+  themes: [githubDark],
+  langs: [],
+  engine: createJavaScriptRegexEngine({ forgiving: true }),
+})
+
+// 同一语言的并发请求共用一次加载
+const langLoads = new Map<string, Promise<boolean>>()
+
+function ensureLanguage(hl: HighlighterCore, lang: string): Promise<boolean> {
+  let load = langLoads.get(lang)
+  if (!load) {
+    const importer = bundledLanguages[lang as keyof typeof bundledLanguages]
+    load = importer
+      ? importer().then((mod) => hl.loadLanguage(mod.default)).then(() => true)
+      : Promise.resolve(false)
+    langLoads.set(lang, load)
+  }
+  return load
+}
+
+async function highlight(req: HighlightRequest): Promise<TokenRun | null> {
+  const hl = await highlighterReady
+  if (!(await ensureLanguage(hl, req.lang))) return null
+  const run: TokenRun = []
+  for (const line of hl.codeToTokensBase(req.code, { lang: req.lang, theme: THEME })) {
+    for (const t of line) {
+      if (!t.content.trim()) continue
+      run.push(t.offset, t.content.length, t.color ?? '', t.fontStyle ?? 0)
+    }
+  }
+  return run
+}
+
+self.addEventListener('message', async (e: MessageEvent<HighlightRequest>) => {
+  const res: HighlightResponse = { id: e.data.id, tokens: await highlight(e.data) }
+  ;(self as unknown as Worker).postMessage(res)
+})