fix(android): lint on the predictive-back opt-in and the shared-art modifier

enableOnBackInvokedCallback only exists from API 33 and the app runs from
31, which lint reports as UnusedAttribute; tools:targetApi says that is
intended, since older releases ignore it. sharedArt becomes an extension
on Modifier, as Compose lint asks of modifier factories.

android/app/src/main/AndroidManifest.xml

@@ -12,6 +12,7 @@
         android:allowBackup="false"
         android:dataExtractionRules="@xml/data_extraction_rules"
         android:enableOnBackInvokedCallback="true"
+        tools:targetApi="33"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
         android:supportsRtl="true"

android/app/src/main/java/io/narl/protonstream/ui/Common.kt

@@ -83,11 +83,11 @@ internal val LocalPaneVisibility = staticCompositionLocalOf<AnimatedVisibilitySc
  */
 @OptIn(ExperimentalSharedTransitionApi::class)
 @Composable
-internal fun sharedArt(key: String): Modifier {
-    val transition = LocalSharedTransition.current ?: return Modifier
-    val visibility = LocalPaneVisibility.current ?: return Modifier
+internal fun Modifier.sharedArt(key: String): Modifier {
+    val transition = LocalSharedTransition.current ?: return this
+    val visibility = LocalPaneVisibility.current ?: return this
     return with(transition) {
-        Modifier.sharedBounds(rememberSharedContentState("art/$key"), visibility)
+        this@sharedArt.sharedBounds(rememberSharedContentState("art/$key"), visibility)
     }
 }
 

android/app/src/main/java/io/narl/protonstream/ui/LibraryScreen.kt

@@ -520,7 +520,7 @@ private fun PosterTile(title: TitleRecord, onClick: () -> Unit, onLongClick: ()
         RemoteArtwork(
             title.posterUrl ?: title.backdropUrl,
             title.canonicalName ?: title.name,
-            Modifier.then(sharedArt(title.key)).fillMaxWidth().aspectRatio(2f / 3f).clip(MaterialTheme.shapes.medium),
+            Modifier.sharedArt(title.key).fillMaxWidth().aspectRatio(2f / 3f).clip(MaterialTheme.shapes.medium),
             fallback = title.thumbnailSource,
         )
         Spacer(Modifier.height(6.dp))

android/app/src/main/java/io/narl/protonstream/ui/TitleScreen.kt

@@ -162,7 +162,7 @@ internal fun TitleScreen(
                 // Edge to edge, fading into the page: the art is the first thing
                 // the page says, and a framed thumbnail with a margin round it read
                 // as one card among the many below it.
-                Box(Modifier.then(sharedArt(title.key)).fillMaxWidth().height(backdropHeight)) {
+                Box(Modifier.sharedArt(title.key).fillMaxWidth().height(backdropHeight)) {
                     RemoteArtwork(
                         title.backdropUrl ?: title.posterUrl,
                         title.canonicalName ?: title.name,