app: cap Coil's parallel fetch and decode work on TV hardware

A sched trace on the TV (release build) caught the real "2fps": not a
slow frame, but a starved one. During navigation the render pipeline
(app RenderThread, SurfaceFlinger/RenderEngine, the mali driver worker,
the Dolby audio service) saturated all four cores together with Coil's
image work — a burst of parallel ~30 ms bitmap decodes split across the
whole IO pool descheduled the main thread hundreds of times and froze
navigation for over a second.

The image loader now caps the request dispatcher at 2 parallel ops and
the decode/transformation pools at 2/1: images still fill in steadily
while scrolling but can no longer eat every core at once. Combined with
the release build on the TV (debug's JIT bursts produced 900 ms+ frames
by themselves), the same navigation settles at p90 ~13 ms per frame
with a worst frame of ~180 ms when a new row composes.

app/src/main/java/net/jeena/jtubetv/JTubeApp.kt

@@ -1,6 +1,7 @@
 package net.jeena.jtubetv
 
 import android.app.Application
+import kotlinx.coroutines.Dispatchers
 import coil.ImageLoader
 import coil.ImageLoaderFactory
 import net.jeena.jtubetv.data.PeerTubeRepository
@@ -33,9 +34,20 @@ class JTubeApp : Application(), coil.ImageLoaderFactory {
      * TVs are slow compositors: decode thumbnails as RGB_565 at capped size so
      * scrolling rows keep a smooth frame rate (user: "the animation has very low fps").
      */
-    override fun newImageLoader(): ImageLoader =
+    override fun newImageLoader(): ImageLoader {
-        ImageLoader.Builder(this)
+        // A sched trace on the TV (release build) showed the main thread being
+        // descheduled hundreds of times during navigation while the render
+        // pipeline (Mali/RenderThread/SF/Dolby) saturated all 4 cores together
+        // with Coil's image work — a burst of parallel 30 ms bitmap decodes
+        // starved the main thread for over a second (the felt "2fps"). Capping
+        // Coil's parallel work: images still fill in steadily while scrolling,
+        // but the decode/fetch pool can no longer eat every core at once.
+        return ImageLoader.Builder(this)
             .bitmapConfig(android.graphics.Bitmap.Config.RGB_565)
             .crossfade(false)
+            .dispatcher(Dispatchers.IO.limitedParallelism(2))
+            .decoderDispatcher(Dispatchers.Default.limitedParallelism(2))
+            .transformationDispatcher(Dispatchers.Default.limitedParallelism(1))
             .build()
+    }
 }