chore(video): correctness and hygiene cleanups in playback layer

Round-up of the small leftovers from the audit. None move the needle on
their own; together they remove a real cancellation bug and tighten the
playback types.

- PlaybackServiceClient.executorService: Executors.newCachedThreadPool()
  → Executors.newSingleThreadExecutor(). The work per callback is
  Future.get() on an already-completed future plus a non-blocking
  trySend; a single thread is plenty. The previous unbounded pool could
  spin up a thread per concurrent video, each lingering for the 60 s
  keep-alive afterwards.

- MediaControllerState.controller: var → val. The field was never
  reassigned anywhere (grep confirms), and a non-observable var on a
  @Stable class is a footgun — Compose can't see writes to a plain var,
  so any future write would silently miss recomposition.

- MediaControllerState.currrentMedia() → currentMedia(). Typo. Updated
  the single caller in PipVideoView.

- LoadThumbAndThenVideoView: real cancellation bug fix. The Coil fetch
  was launched into AccountViewModel.viewModelScope via a side helper
  (loadThumb), so a scroll-away didn't cancel the in-flight image
  request — wasted bandwidth and a late callback writing into stale
  state. Inline the Coil call into the LaunchedEffect's own scope so
  cancellation propagates, and key the effect on thumbUri so a recycled
  audio-track slot with a new cover doesn't stall on the prior
  Pair(true, ...) gate. Drop the now-unused AccountViewModel.loadThumb
  and its only-here imports.

amethyst/src/main/java/com/vitorpamplona/amethyst/service/playback/composable/LoadThumbAndThenVideoView.kt

@@ -29,7 +29,14 @@ import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.layout.ContentScale
 import androidx.compose.ui.platform.LocalContext
+import coil3.asDrawable
+import coil3.imageLoader
+import coil3.request.ImageRequest
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.AccountViewModel
+import com.vitorpamplona.quartz.utils.Log
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.withContext
+import kotlin.coroutines.cancellation.CancellationException
 
 @Composable
 fun LoadThumbAndThenVideoView(
@@ -45,56 +52,47 @@ fun LoadThumbAndThenVideoView(
     accountViewModel: AccountViewModel,
     onDialog: (() -> Unit)? = null,
 ) {
-    var loadingFinished by remember { mutableStateOf<Pair<Boolean, Drawable?>>(Pair(false, null)) }
+    var loadingFinished by remember(thumbUri) { mutableStateOf<Pair<Boolean, Drawable?>>(Pair(false, null)) }
     val context = LocalContext.current
 
-    LaunchedEffect(Unit) {
-        accountViewModel.loadThumb(
-            context,
-            thumbUri,
-            onReady = {
-                loadingFinished =
-                    if (it != null) {
-                        Pair(true, it)
-                    } else {
-                        Pair(true, null)
+    // Run the Coil fetch in this LaunchedEffect's scope (was previously launched into the
+    // AccountViewModel's viewModelScope, which meant a scroll-away wouldn't cancel the in-flight
+    // image request — wasted bandwidth, plus the late callback wrote into a state that no
+    // longer mattered). Keying on thumbUri also makes the effect re-fire when a recycled slot
+    // gets a new audio track with a new cover instead of stalling on the stale Pair(true, ...).
+    LaunchedEffect(thumbUri) {
+        loadingFinished =
+            try {
+                val request = ImageRequest.Builder(context).data(thumbUri).build()
+                val drawable =
+                    withContext(Dispatchers.IO) {
+                        context.imageLoader
+                            .execute(request)
+                            .image
+                            ?.asDrawable(context.resources)
                     }
-            },
-            onError = { loadingFinished = Pair(true, null) },
-        )
+                Pair(true, drawable)
+            } catch (e: Exception) {
+                if (e is CancellationException) throw e
+                Log.e("VideoView", "Fail to load cover $thumbUri", e)
+                Pair(true, null)
+            }
     }
 
     if (loadingFinished.first) {
-        if (loadingFinished.second != null) {
-            VideoView(
-                videoUri = videoUri,
-                mimeType = mimeType,
-                title = title,
-                thumb = VideoThumb(loadingFinished.second),
-                roundedCorner = roundedCorner,
-                contentScale = contentScale,
-                artworkUri = thumbUri,
-                authorName = authorName,
-                nostrUriCallback = nostrUriCallback,
-                isLiveStream = isLiveStream,
-                accountViewModel = accountViewModel,
-                onDialog = onDialog,
-            )
-        } else {
-            VideoView(
-                videoUri = videoUri,
-                mimeType = mimeType,
-                title = title,
-                thumb = null,
-                roundedCorner = roundedCorner,
-                contentScale = contentScale,
-                artworkUri = thumbUri,
-                authorName = authorName,
-                nostrUriCallback = nostrUriCallback,
-                isLiveStream = isLiveStream,
-                accountViewModel = accountViewModel,
-                onDialog = onDialog,
-            )
-        }
+        VideoView(
+            videoUri = videoUri,
+            mimeType = mimeType,
+            title = title,
+            thumb = loadingFinished.second?.let { VideoThumb(it) },
+            roundedCorner = roundedCorner,
+            contentScale = contentScale,
+            artworkUri = thumbUri,
+            authorName = authorName,
+            nostrUriCallback = nostrUriCallback,
+            isLiveStream = isLiveStream,
+            accountViewModel = accountViewModel,
+            onDialog = onDialog,
+        )
     }
 }

amethyst/src/main/java/com/vitorpamplona/amethyst/service/playback/composable/MediaControllerState.kt

@@ -31,14 +31,13 @@ import kotlin.uuid.Uuid
 class MediaControllerState(
     // each composable has an ID.
     val id: String = Uuid.random().toString(),
-    // This is filled after the controller returns from this class
-    var controller: Player,
+    val controller: Player,
     // visibility onscreen
     val visibility: VisibilityData = VisibilityData(),
 ) {
     fun isPlaying() = controller.isPlaying
 
-    fun currrentMedia() = controller.currentMediaItem?.mediaId
+    fun currentMedia() = controller.currentMediaItem?.mediaId
 
     fun toggleMute() {
         controller.volume = if (controller.volume == 0f) 1f else 0f

amethyst/src/main/java/com/vitorpamplona/amethyst/service/playback/pip/PipVideoView.kt

@@ -58,7 +58,7 @@ fun RenderPipVideo(
     val modifier =
         remember {
             val ratio =
-                controller.currrentMedia()?.let {
+                controller.currentMedia()?.let {
                     MediaAspectRatioCache.get(it)
                 }
 

amethyst/src/main/java/com/vitorpamplona/amethyst/service/playback/service/PlaybackServiceClient.kt

@@ -36,7 +36,12 @@ import kotlin.uuid.ExperimentalUuidApi
 import kotlin.uuid.Uuid
 
 object PlaybackServiceClient {
-    val executorService: ExecutorService = Executors.newCachedThreadPool()
+    // Runs the MediaController.buildAsync() completion callbacks. The work per callback is
+    // trivial — Future.get() on an already-completed future plus a non-blocking trySend into
+    // the callbackFlow channel — so a single thread is plenty. The previous newCachedThreadPool
+    // could spin up an unbounded number of threads when many videos appeared at once, each
+    // sticking around for the executor's keep-alive (60s) afterwards.
+    val executorService: ExecutorService = Executors.newSingleThreadExecutor()
 
     fun shutdown() {
         executorService.shutdown()

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/AccountViewModel.kt

@@ -22,7 +22,6 @@ package com.vitorpamplona.amethyst.ui.screen.loggedIn
 
 import android.annotation.SuppressLint
 import android.content.Context
-import android.graphics.drawable.Drawable
 import android.os.Handler
 import android.os.Looper
 import android.util.LruCache
@@ -34,9 +33,6 @@ import androidx.compose.runtime.rememberCoroutineScope
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.ViewModelProvider
 import androidx.lifecycle.viewModelScope
-import coil3.asDrawable
-import coil3.imageLoader
-import coil3.request.ImageRequest
 import com.vitorpamplona.amethyst.AccountInfo
 import com.vitorpamplona.amethyst.Amethyst
 import com.vitorpamplona.amethyst.LocalPreferences
@@ -1578,29 +1574,6 @@ class AccountViewModel(
         super.onCleared()
     }
 
-    fun loadThumb(
-        context: Context,
-        thumbUri: String,
-        onReady: (Drawable?) -> Unit,
-        onError: (String?) -> Unit,
-    ) {
-        viewModelScope.launch(Dispatchers.IO) {
-            try {
-                val request = ImageRequest.Builder(context).data(thumbUri).build()
-                val myCover =
-                    context.imageLoader
-                        .execute(request)
-                        .image
-                        ?.asDrawable(context.resources)
-                onReady(myCover)
-            } catch (e: Exception) {
-                if (e is CancellationException) throw e
-                Log.e("VideoView", "Fail to load cover $thumbUri", e)
-                onError(e.message)
-            }
-        }
-    }
-
     fun loadMentions(
         mentions: ImmutableList<String>,
         onReady: (ImmutableList<User>) -> Unit,