Rancher UI branding customisation is not persistent on the login screen

[aalves08]

Summary

Fixes #10788

• update brand mixin to handle login scenario for branding application of primary color and link color
• add e2e tests to cover possible regression scenario

Occurred changes and/or fixed issues

Technical notes summary

Needed to convert the globalSettings variable in a getter-based in order to get the settings information from the network request done last to settings, which is done from the login view. Had to convert all the watchers to immediate: true in order to pick up changes

Areas or cases that should be tested

• This needs a backend version 2.9-head with date 12th of April 2024 or later, due to changes done in rancher/rancher add missing settings for branding customisation of login/logout page rancher#45108
• Go to Global Settings -> branding and change primary color and link color to custom ones
• Log out of Rancher and check that the branding changes are applied
• Do a refresh on the login page to check that changes persist
• Log back into Rancher and check that general branding changes are applied

Areas which could experience regressions

Screenshot/Video

Checklist

• The PR is linked to an issue and the linked issue has a Milestone, or no issue is needed
• The PR has a Milestone
• The PR template has been filled out
• The PR has been self reviewed
• The PR has a reviewer assigned
• The PR has automated tests or clear instructions for manual tests and the linked issue has appropriate QA labels, or tests are not needed
• The PR has reviewed with UX and tested in light and dark mode, or there are no UX changes

[eva-vashkevich]

I would add a refresh of login page to make sure it stays consistent on refresh, but overall looks good

[eva-vashkevich]

lgtm