Add main menu scene with Rive UI, procedural background, and scene transitions

- Add Menu scene with Rive-driven login and navigation UI
  (RiveLoginController, RiveNavigationController) for entering a
  username and starting/navigating the client via Rive triggers
- Add SceneTransitionController to fade out and load the Landscape
  gameplay scene when Play is selected
- Add procedural low-poly triangle field background and animated wave
  panel (LowPolyTriangleField, TriangleWavePanel) with matching
  editor menus and materials
- Add MenuLightAnimator for animated menu lighting and UI fade helpers
  (MenuFadeIn, DebugFadeToggle/Bridge)
- Rename SampleScene to Landscape and wire ChatNetworkManager to hide
  the new Rive login canvas once gameplay starts
- Bump Input System action codegen version and add Rive package
  references/project settings updates to support the new UI

ProjectSettings/PackageManagerSettings.asset

@@ -18,23 +18,27 @@ MonoBehaviour:
   m_SeeAllPackageVersions: 0
   m_DismissPreviewPackagesInUse: 0
   oneTimeWarningShown: 0
-  oneTimeDeprecatedPopUpShown: 1
-  m_Registries:
-  - m_Id: main
+  oneTimePackageErrorsPopUpShown: 0
+  m_MainRegistry:
+    m_Id: main
     m_Name: 
     m_Url: https://packages.unity.com
     m_Scopes: []
     m_IsDefault: 1
+    m_IsUnityRegistry: 1
     m_Capabilities: 7
     m_ConfigSource: 0
     m_Compliance:
       m_Status: 0
       m_Violations: []
+  m_ScopedRegistries: []
   m_UserSelectedRegistryName: 
   m_UserAddingNewScopedRegistry: 0
   m_RegistryInfoDraft:
     m_Modified: 0
     m_ErrorMessage: 
-    m_UserModificationsInstanceId: -868
-    m_OriginalInstanceId: -870
+    m_UserModificationsEntityId:
+      m_rawData: 568105589213756512
+    m_OriginalEntityId:
+      m_rawData: 568105589213756510
   m_LoadAssets: 0