Use AppBase instead of Application

[willeastcott]

Migrate from Application to AppBase for the pc-app element. This makes it easy to switch on WebGPU support.

This PR also allows the examples to run without throwing exceptions in non-XR capable browsers.

[Copilot]

Pull Request Overview

Migrate the PlayCanvas integration from the old Application API to the new AppBase-plus-createGraphicsDevice/AppOptions pattern, enabling flexible backend switching for WebGPU support.

• Replace all Application references with AppBase in element classes
• Initialize graphics device separately via createGraphicsDevice and use AppOptions for app setup
• Add optional chaining around app.xr in the example script to safely handle missing XR support

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/entity.ts	Switched createEntity and buildHierarchy signatures to accept AppBase instead of Application
src/app.ts	Replaced direct Application creation with createGraphicsDevice + AppOptions + AppBase
examples/js/example.mjs	Guard app.xr calls with optional chaining to avoid null-reference errors

Comments suppressed due to low confidence (2)

src/entity.ts:53

• The JSDoc comment above createEntity still references Application. Please update it to mention AppBase so the docs match the new signature.

createEntity(app: AppBase) {

src/entity.ts:108

• Similarly, update the JSDoc for buildHierarchy to use AppBase instead of Application.

buildHierarchy(app: AppBase) {