add @mm_projext_role variable

[harminius]

Resolves #935

Adds @mm_project_role, a QGIS variable holding the current user's role on the active Mergin Maps project.

This replaces #943, which was reverted in #949. The role is a property of the signed in user, so it is now a global variable and never touches the project file.

Trigger	Function	Where the value comes from	Network
Open or switch project	on_qgis_project_changed	metadata via set_qgis_project_mergin_variables, then refresh_project_role overrules it from the server	1 GET
Open a non-Mergin project	on_qgis_project_changed (else)	cleared	none
Plugin start / sign-in	create_manager	refresh_project_role	1 GET
Status dialog opened	project_status	the project_info response the dialog already fetches	none
Sync with nothing to do	sync_project	metadata, refreshed by the client during project_status()	none
Sync completed	sync_project	metadata, refreshed by the pull/push	none
Project created	create_project	metadata just written from the server response	none
Plugin unloaded	unload	cleared	none
Any of the above while offline	refresh_project_role swallows the error	last synced value from the metadata stands	none