Turn MatchHud into a uGUI canvas binder

Replace the runtime IMGUI (OnGUI) HUD with a binder that drives the authored
MatchHudCanvas widgets via [SerializeField] references (Canvas / Phase /
Time / Crosshair). Visibility toggles Canvas.enabled instead of the
GameObject, so Update keeps polling while hidden; an Awake guard disables
the component on unwired references. Drop the AddComponent<MatchHud>()
bootstrap in MatchManager - the component now lives on the scene canvas.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/Network/MatchManager.cs

@@ -212,9 +212,6 @@ public class MatchManager : MonoBehaviour
             _playerPrefab = _net.NetworkConfig.PlayerPrefab;
             _net.NetworkConfig.PlayerPrefab = null;
         }
-
-        // Placeholder on-screen match HUD, added at runtime so it needs no scene/prefab authoring.
-        if (!GetComponent<MatchHud>()) gameObject.AddComponent<MatchHud>();
     }
 
     private void OnEnable()