Feat/issue 935 mm project role

[harminius]

Resolves #935
Depends on MerginMaps/python-api-client#323

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

The value comes from the project's own metadata, so it resolves offline and costs nothing to read.

• Add @mm_project_role — the variable is written alongside the other mm_project_* variables, and re-read after project_status() (so it follows a role change even when there is nothing to sync), after a completed sync, and when a project is opened or created.
• Refresh project role from the server — asks the server for the current role and stores it, on every project switch and once when the client first appears. Project switching is the main path. The call in create_manager() covers the project QGIS restores at startup, which is read before initGui has built a client to ask with. Prefers project_info_v2, which carries the role without the file list, and falls back to v1

Any failure leaves the stored role alone so an offline start behaves as before.