fix: Update mesh transform on every frame to enable rotation controls

The cube wasn't rotating because the scene was created once with initial
rotation values and never updated. The renderUpdate callback was updating
the rotation variables but not applying them to the actual mesh.

Fixed by:
- Casting rendererInterface to *SceneRenderer in renderUpdate
- Calling UpdateMeshTransform with current rotation values on every frame
- This matches the pattern from the original working implementation

Now both auto-rotation and manual controls (arrow buttons/keys) work
correctly by updating the mesh transform in real-time.

Note: The Guix DSL parser doesn't support complex if statements in
function literals, so this fix is applied directly to the generated
app_gen.go file.

Author: claude

examples/webgpu-cube/app.gx

@@ -1,6 +1,7 @@
 package main
 
 import "fmt"
+import "github.com/gaarutyunov/guix/pkg/runtime"
 
 func App() (Component) {
 	rotationX := 0.0