Add sample for app testing

[oliexe]

Note

Medium Risk
Primarily adds a sample scene/scripts, but also changes Unity EditorBuildSettings and iOS signing configuration, which can affect build behavior in CI or local environments.

Overview
Adds a runnable Unity sample (StashSample.unity) with UI buttons demonstrating Stash native surfaces (OpenCard, OpenModal, OpenBrowser) and a helper (StashLinkGenerator) that calls Stash test API endpoints to generate checkout and pre-authenticated webshop URLs.

Introduces supporting assets under Assets/Resources (including BillingMode.json and app icon metadata) and updates ProjectSettings/EditorBuildSettings.asset to make the new scene the enabled build scene, plus a small ProjectSettings.asset tweak to appleEnableAutomaticSigning.

^{Written by Cursor Bugbot for commit cb92610. This will update automatically on new commits. Configure here.}