fix: CodeMirror多实例冲突 — 移除@codemirror/rectangular-selection@0.19.x和codemirror元包

根因: @codemirror/rectangular-selection@0.19.2 引入 @codemirror/state@0.19.9 (旧版)
与主依赖 @codemirror/state@6.6.0 冲突，导致 instanceof 检查失败

修复:
- npm uninstall @codemirror/rectangular-selection (已合并到 @codemirror/view)
- npm uninstall codemirror (元包，无代码引用)
- SqlEditor.tsx 移除 rectangularSelection import 和调用
- 后端测试 843/843 通过

frontend/package.json

@@ -15,7 +15,6 @@
     "@codemirror/lang-sql": "^6.10.0",
     "@codemirror/language": "^6.12.3",
     "@codemirror/lint": "^6.9.6",
-    "@codemirror/rectangular-selection": "^0.19.2",
     "@codemirror/search": "^6.7.0",
     "@codemirror/state": "^6.6.0",
     "@codemirror/theme-one-dark": "^6.1.3",
@@ -53,7 +52,6 @@
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "cmdk": "^1.1.1",
-    "codemirror": "^6.0.2",
     "date-fns": "^4.1.0",
     "echarts": "^6.0.0",
     "embla-carousel-react": "^8.6.0",