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/editor/index.ts

@@ -0,0 +1,39 @@
+import { defaultKeymap, history, historyKeymap, indentWithTab } from '@codemirror/commands'
+import { markdownKeymap, markdownLanguage } from '@codemirror/lang-markdown'
+import { EditorState, Prec, type Extension } from '@codemirror/state'
+import { drawSelection, EditorView, keymap } from '@codemirror/view'
+import { codeBlockHighlight } from './codeblocks'
+import { livePreview, resolveImageUrl } from './livepreview'
+import { markdownTheme } from './theme'
+
+export interface EditorOptions {
+  parent: HTMLElement
+  doc: string
+  resolveImage?: (src: string) => string
+  onChange?: (state: EditorState) => void
+  extensions?: Extension[]
+}
+
+export function createEditor(opts: EditorOptions): EditorView {
+  const extensions: Extension[] = [
+    history(),
+    drawSelection(),
+    EditorView.lineWrapping,
+    // 不用 lang-markdown 的 markdown()：它会静态引入 lang-html/css/javascript 用于嵌套 HTML 高亮，
+    // 首屏包 gzip 从约 108 KB 增至约 180 KB，而 Live Preview 不需要 HTML 高亮
+    markdownLanguage,
+    Prec.high(keymap.of(markdownKeymap)),
+    markdownTheme,
+    livePreview,
+    codeBlockHighlight,
+    keymap.of([indentWithTab, ...defaultKeymap, ...historyKeymap]),
+    EditorView.contentAttributes.of({ spellcheck: 'false', autocapitalize: 'sentences' }),
+  ]
+  if (opts.resolveImage) extensions.push(resolveImageUrl.of(opts.resolveImage))
+  if (opts.onChange) {
+    const onChange = opts.onChange
+    extensions.push(EditorView.updateListener.of((u) => u.docChanged && onChange(u.state)))
+  }
+  if (opts.extensions) extensions.push(...opts.extensions)
+  return new EditorView({ parent: opts.parent, state: EditorState.create({ doc: opts.doc, extensions }) })
+}