feat(app,android,ios): a blurred poster where a title has no wide art

A title with no fanart, banner or backdrop drew its poster cropped to the
band's shape across the title page and the library's banner: a blown-up
slice of the middle of it, which read as broken. Now the poster is
squeezed to 64x36 and blurred once when it is fetched, the cache keeps
that wash, and it is stretched across the band. The library's banner
stands the sharp poster at its right over it. Android blurs the poster
it already had (Android 12 and later; older versions decode it tiny),
and iOS likewise.

METADATA.md also records the quality badges of the commit before.

ios/ProtonStream/UI/LibraryView.swift

@@ -494,7 +494,7 @@ private struct FeaturedBanner: View {
     let onOpen: () -> Void
 
     var body: some View {
-        RemoteArtwork(title.backdropUrl ?? title.posterUrl, title.displayName, fallback: title.thumbnailSource, labelled: false)
+        Backdrop(title: title)
             .aspectRatio(4 / 3, contentMode: .fit)
             .overlay {
                 LinearGradient(

ios/ProtonStream/UI/RemoteArtwork.swift

@@ -81,6 +81,19 @@ struct RemoteArtwork: View {
     }
 }
 
+/// A title's art across the top of a page: its backdrop, or with none, its
+/// poster as a wash of its own colours — blurred, so it reads as the poster's
+/// light rather than a blown-up slice of it. The desktop's `Fit::Ambient`.
+struct Backdrop: View {
+    let title: TitleRecord
+
+    var body: some View {
+        let wash = title.backdropUrl == nil && title.posterUrl != nil
+        RemoteArtwork(title.backdropUrl ?? title.posterUrl, title.displayName, fallback: title.thumbnailSource, labelled: false)
+            .blur(radius: wash ? 24 : 0, opaque: true)
+    }
+}
+
 /// A title's logo — its name as lettering — in place of `fallback`, once it
 /// has loaded. `fallback` stands until then, and for good when there is none,
 /// so a title without a logo looks as it always did and one with a logo only

ios/ProtonStream/UI/TitleView.swift

@@ -119,7 +119,7 @@ struct TitleView: View {
     /// Edge to edge, fading into the page: the art is the first thing the page
     /// says.
     private func backdrop(height: CGFloat, top: CGFloat) -> some View {
-        RemoteArtwork(title.backdropUrl ?? title.posterUrl, title.displayName, fallback: title.thumbnailSource, labelled: false)
+        Backdrop(title: title)
             .frame(height: height)
             .frame(maxWidth: .infinity)
             .clipped()