Add 'Set UI Size to Default' to help menu on login screen

[akleshchev]

Description

Issue:

User can set scale to be too large with no obvious way out of it. We have a shortcut inworld to fix it. But not only login button will be out of view, option is not visible due to being under 'advanced' menu inworld, while user is likely stuck on the login.

Task:

Dupplicate the option from 'advanced' inworld menu to 'help' menu on login screen.

Test plan:

1. Set UI scale to some large value
2. Click on menu's Help > Set UI Size to Default
3. Verify Scale resets to 1.0

[zp6]

PR submitted: #5823

Wallet: zp6

[jellyfish-linden]

Passed QA.
Verified on Second Life Release 26.4.0.33417795895 on Win/OSX in the scope of https://github.com/secondlife/iqa/issues/604