Feature: Improve/Align media playback modes

[levitatingpineapple]

Scope

While the app does not support media enclosures, the feeds can already include media in their HTML content.
This issue aims to align the media playback experience between platforms by enabling OS provided features.

Current Support

	iOS/iPadOS	macOS
Fullscreen	✅	❌
Picture in Picture	✅	❌
Background playback	❌*	n/a

• it's possible to trick iOS into playing in background by first enabling PiP and then restarting playback from lockscreen.

Enabling macOS Fullscreen

This can be easily achieved by setting isElementFullscreenEnabled.
Would require some testing though - multiple monitors, window resizing etc.

Enabling macOS PiP

While in maOS allowspictureinpicturemediaplayback is marked as private - this property has been available since iOS9, and is also enabled in macOS Safari since Sierra, making it quite unlikely to be changed/removed.

Enabling iOS Background Playback

Add audio to the background modes capability.