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/src/components/SqlEditor.tsx

@@ -13,7 +13,7 @@ import {
 } from '@codemirror/language';
 import { closeBrackets, closeBracketsKeymap } from '@codemirror/autocomplete';
 import { searchKeymap, highlightSelectionMatches } from '@codemirror/search';
-import { rectangularSelection } from '@codemirror/rectangular-selection';
+
 
 // ---------------------------------------------------------------------------
 // Types
@@ -296,7 +296,6 @@ const SqlEditor: React.FC<SqlEditorProps> = ({
         bracketMatching(),
         closeBrackets(),
         highlightSelectionMatches(),
-        rectangularSelection(),
         syntaxHighlighting(defaultHighlightStyle, { fallback: true }),
         appTheme,
         oneDark,