Allow loading of in-app content

This can improve the user experience, reduce bandwidth usage, and increase security.

**Possible implementations**
- Electron: [`webContents.loadFile`](https://www.electronjs.org/docs/latest/api/web-contents#contentsloadfilefilepath-options)
- Android: [`WebViewAssetLoader`](https://developer.android.com/develop/ui/views/layout/webapps/load-local-content#java)
- iOS: -

No files, tests, or entry points are named. Compare the Electron, Android, and iOS platform implementations to determine how bundled in-app content should be loaded on each platform. The work is done when the intended behavior and platform-specific scope are defined and verified consistently across the supported implementations.

Written by the indexing model from the issue text.