Update clients, docs, and install for CarConnectivity migration

- gui_client: update _TRACE_LABELS paths to new flat snapshot structure
- README: update library name, pip command, domain list, credentials
  template, output example, and module reference
- install.sh: update credentials template and systemd service description
- client.py: update docstring
- data_model: Kelvin→Celsius conversion for battery temps (CarConnectivity
  returns Kelvin); round range_estimated_full to 1 decimal
- jay_diff: fix merge when old value is not a dict; skip empty-dict
  branches in delete

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

utils/jay_diff.py

@@ -77,7 +77,10 @@ def jay_merge_update(_old, _new):
 			r = jay_merge_update(_old.get(k, {}), v)
 			_old[k] = r
 		else:
-			_old[k] = _new[k]
+			if isinstance(_old, dict):
+				_old[k] = _new[k]
+			else:
+				_old = _new
 	return _old
 
 
@@ -96,7 +99,7 @@ def jay_merge_add(_old, _new):
 
 def jay_merge_delete(_old, _new):
 	for k, v in _new.items():
-		if isinstance(v, dict):
+		if isinstance(v, dict) and v:
 			jay_merge_delete(_old[k], _new[k])
 			if len(_old[k].keys()) == 0:
 				del _old[k]