Commit e63584843 declared applicationFilePath inside the WINDOWS_SYS
block but uses it unconditionally to fill conf.main_executable_path,
so every non-Windows GUI build fails with 'applicationFilePath was
not declared in this scope'. Declare it at function scope, seeded
from argv[0] (the previous non-Windows behaviour); Windows still
overrides it with QCoreApplication::applicationFilePath().
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>