Fix new-user first-login tweak context and script prep

[jacksyrm]

Questions

• Did you test your changes or double-check that they work?
• Did you read and follow the Atlas Contribution Guidelines?

Describe your pull request

• ensure first-login/new-user tweaks run in the correct user context instead of elevating into a different profile
• add a shared prepareSetting.cmd helper and update related AtlasDesktop scripts to reuse consistent elevation/state handling
• reduce noisy registry/user enumeration errors and make new-user setup more resilient (including lockscreen failure handling)

[TheyCreeper]

lgtm