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

backend/src/modules/dc/tool-c/routes/index.ts

@@ -61,6 +61,11 @@ export async function toolCRoutes(fastify: FastifyInstance) {
     handler: sessionController.updateHeartbeat.bind(sessionController),
   });
 
+  // ✨ 获取列的唯一值（用于数值映射）
+  fastify.get('/sessions/:id/unique-values', {
+    handler: sessionController.getUniqueValues.bind(sessionController),
+  });
+
   // ==================== AI代码生成路由（Day 3） ====================
 
   // 生成代码（不执行）