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/vite.config.ts

@@ -0,0 +1,6 @@
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  worker: { format: 'es' },
+  test: { environment: 'node' },
+})