marker: persist project-wide markers_paused to the .gns3 file

pause/resume was fire-and-forget: the controller sent marker pause/resume but
stored nothing, so the Web UI could only keep a local optimistic flag that was
lost on panel reopen. Treat the mute as a project-level config (like per-marker
enabled): record _markers_paused, persist it in the topology (asdict + load),
and echo it on the project object so the UI renders from server truth.

Because marker pause is a uBridge runtime flag that resets on node restart,
start_all re-applies the mute to freshly started uBridges after a project
reopen — a paused project stays paused across close/reopen.

tests/controller/test_project.py

@@ -83,6 +83,7 @@ async def test_json():
         "supplier": None,
         "variables": None,
         "marker_definitions": {},
+        "markers_paused": False,
         "created_by": None
     }