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/EditorBuildSettings.asset

@@ -6,8 +6,10 @@ EditorBuildSettings:
   serializedVersion: 2
   m_Scenes:
   - enabled: 1
-    path: Assets/Scenes/SampleScene.unity
+    path: Assets/Scenes/Landscape.unity
     guid: 99c9720ab356a0642a771bea13969a05
+  - enabled: 1
+    path: Assets/Scenes/Menu.unity
+    guid: f4691f683da62244ebc443c6c81fb9fc
   m_configObjects:
     com.unity.input.settings.actions: {fileID: -944628639613478452, guid: 052faaac586de48259a63d0c4782560b, type: 3}
-  m_UseUCBPForAssetBundles: 0