NSIS installer build — corrected source paths

Extract the ENTIRE ZIP into a normal folder first. Do not copy only Installer.
The package root must contain BOTH aurexion and Installer folders.
Open Installer and run BUILD-INSTALLER.bat.
The EXE is created in the package root, next to the aurexion folder.

The BAT now validates the application payload and passes an explicit absolute
PACKAGE_ROOT to NSIS. The NSI also resolves paths from its own file directory
when compiled directly. Paths containing spaces are supported.

NSIS 3 is required only to rebuild the installer. The application requires XAMPP.
Compiler checks passed here; Windows runtime installation remains untested here.
