chore: 编译产物不再入库，启动脚本改用 server.exe

- server.exe 早已写在 .gitignore 第 9 行，却仍被追踪，规则一直没生效（.gitignore
  只管未被追踪的文件）。git rm --cached 移出索引，本地文件保留
- 启动项目.bat 改为启动前先 go build 重建再运行 server.exe，编译失败就停住不启动：
  原来用 go run 正是为了不跑到过期的 exe，换成 exe 后得靠这一步补上

启动项目.bat

@@ -3,9 +3,19 @@ title Warehouse Management System Launcher
 
 cd /d "%~dp0"
 
+echo Building backend...
+rem Rebuild on every launch so the executable can never be a stale one.
+go build -o server.exe ./cmd/server
+if errorlevel 1 (
+  echo.
+  echo Backend build failed. Fix the errors above and run this launcher again.
+  pause
+  exit /b 1
+)
+
 echo Starting backend (8082)...
-rem Run the current source so local testing never uses an outdated server.exe.
-start "Warehouse MMS - Backend" /D "%~dp0" cmd /k "go run ./cmd/server"
+rem Work dir must stay at the project root: frontend/dist and uploads are relative paths.
+start "Warehouse MMS - Backend" /D "%~dp0" cmd /k "server.exe"
 
 echo Starting frontend (8083)...
 start "Warehouse MMS - Frontend" /D "%~dp0frontend" cmd /k "npm run dev"