OpenBackup v1.2.0

Distribution milestone.\n\n- Added a self-contained Windows EXE to the release package.\n- Added Install.ps1 with desktop shortcut creation.\n- Added Uninstall.ps1 that leaves backup snapshots untouched.\n- Included source code separately for open-source review.\n- Verified application build, backup, restore, scheduling, retention, and launch behavior.\n- Included grouped installer package OpenBackup-012.