clear the compile warnings the downloads work left behind

- ArtworkStore's fetcher still did `response.body ?: throw` from before the body
  size limit landed; OkHttp's `body` is not nullable, so the elvis was dead.
- DownloadRepository's `downloadStates` uses `flatMapLatest` without the opt-in
  its callers annotate.
- HeroHeader's subtitle tap invoked `onSubtitleClick?.invoke()` inside the branch
  where it is already known to be non-null.
- MediaItemTest used a safe call on `clippingConfiguration`, which never returns
  null.

No behaviour change: a clean full rebuild now compiles without warnings.

app/src/main/kotlin/com/subtracks/data/download/ArtworkStore.kt

@@ -85,7 +85,7 @@ class OkHttpArtworkFetcher(
             val request = Request.Builder().url(url).build()
             http.newCall(request).execute().use { response ->
                 if (!response.isSuccessful) throw IOException("Artwork request failed with ${response.code}")
-                val body = response.body ?: throw IOException("Artwork response had no body")
+                val body = response.body
                 if (body.contentLength() > maxBytes) throw IOException("Artwork response is larger than $maxBytes bytes")
                 val source = body.source()
                 source.request(maxBytes + 1)

app/src/main/kotlin/com/subtracks/data/repo/DownloadRepository.kt

@@ -16,6 +16,7 @@ import kotlinx.coroutines.CompletableDeferred
 import kotlinx.coroutines.CoroutineDispatcher
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.Job
 import kotlinx.coroutines.SupervisorJob
 import kotlinx.coroutines.cancel
@@ -157,6 +158,7 @@ class DownloadRepository(
             resumed.forEach { fetchArtworkSafely(it.sourceId, it.songId) }
         }
 
+    @OptIn(ExperimentalCoroutinesApi::class)
     private fun downloadStates(): Flow<Map<String, SongDownload>> =
         sourceRepository
             .activeSourceId()

app/src/main/kotlin/com/subtracks/ui/components/HeroDetailScaffold.kt

@@ -253,7 +253,7 @@ fun HeroHeader(
                 modifier =
                     Modifier
                         .heightIn(min = 32.dp)
-                        .clickable(onClickLabel = "Open artist") { onSubtitleClick?.invoke() },
+                        .clickable(onClickLabel = "Open artist") { onSubtitleClick() },
                 contentAlignment = Alignment.TopCenter,
             ) {
                 HeroSubtitle(subtitle, Modifier.padding(top = 4.dp))