fix: verify PySide6 install with pip show instead of exit code

pip install can return non-zero even on success (e.g. deprecation
warnings). Use pip show after install to confirm the package is
actually available.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hanshino

start.bat

@@ -21,6 +21,7 @@ cd /d "%_root%"
 if %errorlevel% neq 0 (
     echo Installing PySide6, please wait...
     "%_root%toolkit\python\python.exe" -m pip install PySide6
+    "%_root%toolkit\python\python.exe" -m pip show PySide6 >nul 2>&1
     if %errorlevel% neq 0 (
         echo.
         echo ERROR: Failed to install PySide6. Cannot start.