Two fixes for critical Windows issues (#81):
1. NSIS installer: disable allowToChangeInstallationDirectory to prevent
users from installing into shared directories where uninstall would
delete all their files. Also set deleteAppDataOnUninstall: false to
preserve user data.
2. Windows process cleanup: killServer() used SIGTERM which Windows
doesn't support, leaving zombie processes that block updates.
Now uses taskkill /T /F to kill the entire process tree on Windows,
ensuring no residual processes remain after app exit.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>