fix(web): schedule saves, restart_required, health and current-status agree with the rig (#750)

- POST /api/v3/config/schedule and /config/dim-schedule accept a disabled per-day schedule with every day off, and keep an off day's times.
- POST /api/v3/config/main answers restart_required only when the save changed a setting the running display does not apply live.
- GET /api/v3/health reports degraded with checks.display_loop.status stopped when the display service is stopped.
- GET /api/v3/display/current-status answers unknown (null fields) after the display stops instead of the cached last state. New web_interface.display_state.display_gone().

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/web_interface/test_web_process_runs_no_plugin_code.py

@@ -221,7 +221,9 @@ class TestTheWebProcessNeverRunsAPlugin:
     def test_saving_its_section_through_the_main_config(self, web):
         body = web.post("/api/v3/config/main", {PLUGIN_ID: {"message": "via main"}})
         assert web.stored()["message"] == "via main"
-        assert body["restart_required"] is True
+        # The display's config watcher hands the section to the running
+        # plugin (on_config_change), as for /plugins/config: no restart.
+        assert body["restart_required"] is False
         assert web.ran() == []
 
     def test_resetting_its_config(self, web):