fix: 固定开发环境后端数据目录，避免 cwd 不同导致头像 404

从 cwd 向上解析 backend 根，并写入本地开发启动约定。

Co-authored-by: Cursor <cursoragent@cursor.com>

start.bat

@@ -25,7 +25,7 @@ if not exist "backend\app.ini" (
 )
 if not exist "backend\data" mkdir "backend\data"
 
-echo   Starting backend (port 3001)...
+echo   Starting backend (port 3001, cwd=backend, data=backend\data)...
 start "jiang13-backend" cmd /k "cd /d %~dp0backend && go run ./cmd/jiang13"
 echo [OK] Backend started
 echo.
@@ -54,6 +54,7 @@ echo   All services started!
 echo.
 echo   Frontend:  http://localhost:3000
 echo   Backend:   http://localhost:3001
+echo   Data dir:  %~dp0backend\data
 echo   Database:  localhost:5432
 echo.
 echo   Close each window to stop the service.