refactor(编辑器): 去掉 vue-codemirror，代码编辑器、协作编辑器、教程编辑器改用自己的薄外壳

三个编辑器只用到它的 v-model、extensions、只读、缩进、占位文字、尺寸和 ready，
shared/components/CodeMirror.vue 照它的做法写了这几样，功能和手感不变：
- 它默认带的 basicSetup（行号、撤销、折叠、搜索、括号……）照样带上
- 各项配置的先后顺序和它一致；外部改代码仍是一次不带 userEvent 的 changes，
  提交时的编辑过程统计不受影响
- @codemirror/commands 现在直接 import，显式声明进依赖

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

apps/web/package.json

@@ -12,6 +12,7 @@
   },
   "dependencies": {
     "@codemirror/autocomplete": "^6.20.3",
+    "@codemirror/commands": "^6.10.4",
     "@codemirror/lang-cpp": "^6.0.3",
     "@codemirror/lang-markdown": "^6.5.2",
     "@codemirror/lang-python": "^6.2.1",
@@ -47,7 +48,6 @@
     "pinyin-pro": "^3.29.5",
     "vue": "^3.5.43",
     "vue-chartjs": "^5.3.4",
-    "vue-codemirror": "^6.1.1",
     "vue-router": "^5.4.0",
     "y-codemirror.next": "^0.3.6",
     "y-protocols": "1.0.7",