V16-RC: Login screen cannot load public manifests

[iOvergaard]

Description

This fixes two issues:

1. Public manifests were no longer being loaded on the login screen as outlined in V16.0.0-RC4: Custom login greeting no longer working #19448, so this fixes V16.0.0-RC4: Custom login greeting no longer working #19448
2. It was not possible to overwrite en-US-specific localizations

Changes

• Registers required contexts for UmbServerExtensionRegistrator: With V16, it now needs the UmbServerContext
• Changes the weight of all login localizations, so that you can overwrite them as extensions by default have weight=0
• Changes the culture of login localizations to match the Backoffice itself, i.e., no country codes: This way you can overwrite Backoffice and Login localizations within one file (this did not work in V15 due to Localization fallback issue #19227 but was fixed already in RC2)

[Copilot]

Pull Request Overview

This PR addresses issues with the login screen by updating localization configurations and ensuring proper server context registration for public manifests.

• Updates localization weights, aliases, and cultures to facilitate proper overriding of login localizations.
• Introduces the required UmbServerContext for the slim backoffice and refines error handling in extension registration.
• Adjusts server extension registrator methods to disable notifications during manifest retrieval.

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

File	Description
src/Umbraco.Web.UI.Login/src/localization/manifests.ts	Updates localization aliases, names, JS imports, and culture codes to remove country codes and adjust weights.
src/Umbraco.Web.UI.Login/src/localization/lang/en-us.ts	Minor formatting and string adjustments for localization content.
src/Umbraco.Web.UI.Login/src/controllers/slim-backoffice-initializer.ts	Introduces server context and revises icon registry attachment while improving error logging.
src/Umbraco.Web.UI.Client/src/libs/extension-api/controller/server-extension-registrator.controller.ts	Adds disableNotifications option in manifest retrieval methods.

[leekelleher]

Tested out, works as expected. 🚀