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>

package.json

@@ -14,14 +14,9 @@
   "license": "ISC",
   "devDependencies": {
     "prettier": "^3.9.6",
-    "vitepress": "1.6.4"
+    "vitepress": "2.0.0-alpha.20"
   },
   "dependencies": {
-    "@codemirror/lang-cpp": "^6.0.3",
-    "@codemirror/lang-python": "^6.2.1",
-    "axios": "^1.20.0",
-    "codemirror": "^6.0.2",
-    "markdown-it-mathjax3": "^5.2.0",
-    "vue-codemirror6": "~1.3.22"
+    "markdown-it-mathjax3": "^4.3.2"
   }
 }