[Feature] Admin-managed announcement banners in the web and mobile apps

I have searched the existing feature requests, both open and closed, to make sure this is not a duplicate request. • Yes The feature I would like server administrators to be able to publish a short announcement that users can see inside both the web interface and the official mobile apps, including after login while browsing their library. Use case My Immich database, thumbnails and previews are stored on a Raspberry Pi, while the original photos and videos are stored on external storage (DAS), which is sometimes switched off because it is noisy and consumes extra power. Scripts on my raspberry automatically moves the new photos to the DAS when connected. I imagine similar configurations may be fairly common in personal or family setups. The downside is that, remaining Immich accessible even with DAS switched off, users can browse the available previews in low quality without realizing that the originals are temporarily unavailable. A banner could explain this cle…