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/index.tsx

@@ -251,8 +251,19 @@ const ToolC = () => {
             onComputeClick={() => updateState({ computeDialogVisible: true })}
             onPivotClick={() => updateState({ pivotDialogVisible: true })}
           />
-          <div className="flex-1 p-4 overflow-hidden">
-            <DataGrid data={state.data} columns={state.columns} />
+          <div className="flex-1 p-4 overflow-hidden flex flex-col">
+            {/* ✨ 优化：提示只显示前50行 */}
+            {state.data.length > 0 && (
+              <div className="mb-2 px-3 py-2 bg-blue-50 border border-blue-200 rounded-lg flex items-center gap-2 text-sm">
+                <span className="text-blue-600">ℹ️</span>
+                <span className="text-blue-700">
+                  <strong>提示：</strong>表格仅展示前 <strong>50行</strong> 数据预览，导出功能将包含 <strong>全部</strong> 处理结果
+                </span>
+              </div>
+            )}
+            <div className="flex-1 overflow-hidden">
+              <DataGrid data={state.data} columns={state.columns} />
+            </div>
           </div>
         </div>