[Feature] background playback and picture-in-picture for videos on mobile

I have searched the existing feature requests, both open and closed, to make sure this is not a duplicate request. • Yes The feature Add an opt-in setting to the mobile app so videos keep playing when the app leaves the foreground. Two related options: 1. Background audio: the audio keeps playing when the app is in the background or the screen is locked, with play/pause controls on the lock screen. 2. Picture-in-picture: when you leave the app during playback (home gesture, app switch), the video shrinks into the system's floating PiP window instead of stopping. Supported natively on Android 8+ and iOS 14+. Why I store music videos and other entertainment media in Immich and want to listen to them while using other apps or with the screen off. Current behavior Playback stops as soon as the app leaves the foreground, so the only way to keep listening is to keep the Immich app open with the screen on. That drains the battery and blocks the phone for anythin…