Fix readonly UI for create document user permission

[madsrasmussen]

Fixes: #19286

Please ensure that the following user permission combinations work as expected:

• Create + Update
• Create
• Update

[Copilot]

Pull Request Overview

This PR refactors how create/update permissions are enforced by removing individual flags, introducing unified handlers, and reacting to the document's isNew state.

• Replaced #userCanCreate/#userCanUpdate flags with #enforceUserPermission method
• Added #handleUserPermissionChange to centralize read-only guard logic
• Observes isNew to decide whether to apply create or update permission rules

Comments suppressed due to low confidence (1)

src/Umbraco.Web.UI.Client/src/packages/documents/documents/workspace/document-workspace.context.ts:192

• Add a JSDoc comment for #enforceUserPermission (and similarly for #handleUserPermissionChange) to explain its purpose, parameters, and side-effects on the read-only guard.

#enforceUserPermission(verb: string, message: string) {

[warrenbuckley]

From an outsider trying to use the readOnly guard, I only stumbled across this PR and realised I had to set permitted to true as opposed to false like the propertyWriteGuard

Would it best if this could be aligned like the others, just to avoid confusion.
And another comment is that the docs page mentions not to use readonlyGuard. Is this the case or can us package devs use it ?

[madsrasmussen]

We already relied on it in core, and to keep this fix as simple as possible, I just corrected the code. We want to move away from it in core as well, and the current plan is for it to be deprecated and removed eventually. If you can solve your case with the view/write guards, I recommend using them. If you can't, please let us know.

As mentioned in the comment, the DX is a bit backward compared to the view/write guards. We want guards to grant access to something instead of removing it.