fix(dc/tool-c): Fix special character handling and improve UX

Major fixes:
- Fix pivot transformation with special characters in column names
- Fix compute column validation for Chinese punctuation
- Fix recode dialog to fetch unique values from full dataset via new API
- Add column mapping mechanism to handle special characters

Database migration:
- Add column_mapping field to dc_tool_c_sessions table
- Migration file: 20251208_add_column_mapping

UX improvements:
- Darken table grid lines for better visibility
- Reduce column width by 40% with tooltip support
- Insert new columns next to source columns
- Preserve original row order after operations
- Add notice about 50-row preview limit

Modified files:
- Backend: SessionService, SessionController, QuickActionService, routes
- Python: pivot.py, compute.py, recode.py, binning.py, conditional.py
- Frontend: DataGrid, RecodeDialog, index.tsx, ag-grid-custom.css
- Database: schema.prisma, migration SQL

Status: Code complete, database migrated, ready for testing

frontend-v2/src/modules/dc/pages/tool-c/components/RecodeDialog.tsx

@@ -41,35 +41,55 @@ const RecodeDialog: React.FC<RecodeDialogProps> = ({
   const [loading, setLoading] = useState(false);
   const [extracting, setExtracting] = useState(false);
 
-  // 当选择列时，提取唯一值
+  // 当选择列时，从后端获取唯一值
   useEffect(() => {
-    if (!selectedColumn || !data || data.length === 0) {
+    if (!selectedColumn || !sessionId) {
       setUniqueValues([]);
       setMappingTable([]);
       return;
     }
 
-    setExtracting(true);
+    const fetchUniqueValues = async () => {
+      setExtracting(true);
+      
+      try {
+        // ✨ 调用后端API获取唯一值（从完整数据中提取，不受前端50行限制）
+        const response = await fetch(
+          `/api/v1/dc/tool-c/sessions/${sessionId}/unique-values?column=${encodeURIComponent(selectedColumn)}`
+        );
+        
+        const result = await response.json();
+        
+        if (!result.success) {
+          throw new Error(result.error || '获取唯一值失败');
+        }
+        
+        const unique = result.data.uniqueValues;
+        
+        setUniqueValues(unique);
+        
+        // 初始化映射表
+        const initialMapping = unique.map((val: any) => ({
+          originalValue: val,
+          newValue: '',
+        }));
+        
+        setMappingTable(initialMapping);
+        
+        // 生成默认新列名
+        setNewColumnName(`${selectedColumn}_编码`);
+      } catch (error: any) {
+        console.error('[RecodeDialog] 获取唯一值失败:', error);
+        message.error(error.message || '获取唯一值失败');
+        setUniqueValues([]);
+        setMappingTable([]);
+      } finally {
+        setExtracting(false);
+      }
+    };
     
-    // 提取唯一值
-    const values = data.map((row) => row[selectedColumn]);
-    const unique = Array.from(new Set(values)).filter(v => v !== null && v !== undefined && v !== '');
-    
-    setUniqueValues(unique);
-    
-    // 初始化映射表
-    const initialMapping = unique.map((val) => ({
-      originalValue: val,
-      newValue: '',
-    }));
-    
-    setMappingTable(initialMapping);
-    
-    // 生成默认新列名
-    setNewColumnName(`${selectedColumn}_编码`);
-    
-    setExtracting(false);
-  }, [selectedColumn, data]);
+    fetchUniqueValues();
+  }, [selectedColumn, sessionId, message]);
 
   // 更新映射值
   const updateMapping = (originalValue: any, newValue: string) => {