Replace the installer-written breadcrumb discriminator with a GUI-side
check. netbirdFootprintExists inspects the daemon config/state files
(default.json, legacy config.json, state.json) under profilemanager's
default config dir; combined with whether the UI preferences file already
existed, this tells a genuinely fresh machine from an existing or
upgrading user. Only the signed GUI, via Wails, ever enables
launch-on-login, and a user's later manual disable is never overridden.
The preferences store now exposes ExistedAtLoad and the --post-update
flag is dropped.
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