remove judge0 code editor, upgrade vitepress to v2

- Drop the <editor> component, Judge0 client and CodeMirror themes/deps
- Pin markdown-it-mathjax3 to ^4 to satisfy vitepress peer dependency
- Upgrade vitepress 1.6.4 -> 2.0.0-alpha.20

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.vitepress/theme/index.ts

@@ -2,13 +2,11 @@ import type { Theme } from "vitepress"
 import DefaultTheme from "vitepress/theme"
 import BVideo from "./components/BVideo.vue"
 import Author from "./components/Author.vue"
-import CodeEditor from "./components/CodeEditor.vue"
 
 export default {
   extends: DefaultTheme,
   async enhanceApp({ app }) {
     app.component("bvideo", BVideo)
     app.component("author", Author)
-    app.component("editor", CodeEditor)
   },
 } satisfies Theme