fix: Batch script syntax error in build-client.bat

- Replace nested if statements with goto for target validation
- Remove parentheses from echo statements that conflict with if blocks

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

scripts/build-client.bat

@@ -402,16 +402,17 @@ echo %BLUE%[STEP]%NC% 打包 Electron 應用...
 cd /d "%CLIENT_DIR%"
 
 REM 驗證 BUILD_TARGET
-if /i not "%BUILD_TARGET%"=="nsis" if /i not "%BUILD_TARGET%"=="portable" (
+if /i "%BUILD_TARGET%"=="nsis" goto :valid_target
+if /i "%BUILD_TARGET%"=="portable" goto :valid_target
 echo %RED%[ERROR]%NC% 無效的打包目標: %BUILD_TARGET%
 echo %BLUE%[INFO]%NC% 有效選項: nsis, portable
 exit /b 1
-)
 
+:valid_target
 if /i "%BUILD_TARGET%"=="nsis" (
-    echo %BLUE%[INFO]%NC% 目標平台: Windows (NSIS 安裝檔) - 推薦
+    echo %BLUE%[INFO]%NC% 目標平台: Windows NSIS 安裝檔 - 推薦
 ) else (
-    echo %BLUE%[INFO]%NC% 目標平台: Windows (Portable)
+    echo %BLUE%[INFO]%NC% 目標平台: Windows Portable
     echo %YELLOW%[WARN]%NC% 注意: Portable 模式的臨時資料夾會在關閉時清空
     echo %YELLOW%[WARN]%NC% SQLite 資料庫已自動儲存到 %%APPDATA%%\Meeting-Assistant
 )