ui: cap the eased scroll and drop the snapshot focus map

A system trace on the TV showed the eased focus scroll re-ticking for
hundreds of milliseconds per move — animateScrollToItem's default spec
scales with distance, and every tick redraws the whole window on a TV
GPU, with 145 ms spikes when new cards composed mid-animation. That is
what drags row scrolling back down to a few fps, again (same symptom
51077b8 and 5ea1ca5 each fixed a facet of). easedScrollToItem keeps the
eased feel but caps the tween at ~140 ms, so a move costs a handful of
cheap ticks instead of hundreds.

The per-card focus requesters in the video grid were back in a snapshot
state map written during composition — the exact recompose loop
5ea1ca5 fixed in the rows, still alive in the grid screens. Plain
HashMap again, keyed on epoch only: keyed on items.size the map was
dropped wholesale on every page load, and already-composed cards didn't
re-populate it until their next recompose.

app/src/main/java/net/jeena/jtubetv/ui/screens/ChannelsScreen.kt

@@ -2,6 +2,8 @@ package net.jeena.jtubetv.ui.screens
 
 import androidx.compose.foundation.BorderStroke
 import androidx.compose.foundation.background
+import androidx.compose.animation.core.tween
+import androidx.compose.foundation.gestures.animateScrollBy
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
@@ -469,7 +471,7 @@ private fun ChannelRow(
             val notFullyVisible = target == null ||
                 target.offset < 0 ||
                 target.offset + target.size > rowState.layoutInfo.viewportEndOffset
-            if (notFullyVisible) rowState.animateScrollToItem(focusedIndex)
+            if (notFullyVisible) rowState.easedScrollToItem(focusedIndex)
         }
     }
     if (focused) {

app/src/main/java/net/jeena/jtubetv/ui/screens/HomeRows.kt

@@ -1,5 +1,7 @@
 package net.jeena.jtubetv.ui.screens
 
+import androidx.compose.animation.core.tween
+import androidx.compose.foundation.gestures.animateScrollBy
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
@@ -9,6 +11,7 @@ import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.width
 import androidx.compose.foundation.lazy.LazyColumn
+import androidx.compose.foundation.lazy.LazyListState
 import androidx.compose.foundation.lazy.LazyRow
 import androidx.compose.foundation.lazy.itemsIndexed
 import androidx.compose.foundation.lazy.rememberLazyListState
@@ -450,7 +453,7 @@ fun FocusScrollRow(
             val notFullyVisible = target == null ||
                 target.offset < 0 ||
                 target.offset + (target.size / 2) > rowState.layoutInfo.viewportEndOffset
-            if (notFullyVisible) rowState.animateScrollToItem(focusedIndex)
+            if (notFullyVisible) rowState.easedScrollToItem(focusedIndex)
         }
     }
     LazyRow(
@@ -495,3 +498,19 @@ fun FocusScrollRow(
         }
     }
 }
+
+/**
+ * Eased scroll to [index] with a capped duration (perf): [animateScrollToItem]'s
+ * default spec keeps re-ticking for hundreds of ms on long distances, and every
+ * tick is a full-window redraw on a TV GPU — which is what dragged the scroll
+ * down to a few fps. A short tween keeps the eased feel at a fraction of the
+ * ticks. (Shared by the home rows and the channels rows.)
+ */
+suspend fun LazyListState.easedScrollToItem(index: Int) {
+    val info = layoutInfo
+    val first = info.visibleItemsInfo.firstOrNull() ?: return
+    val stride = first.size + info.mainAxisItemSpacing
+    if (stride <= 0) return
+    val delta = index * stride - (first.index * stride + firstVisibleItemScrollOffset)
+    animateScrollBy(delta.toFloat(), tween(140))
+}

app/src/main/java/net/jeena/jtubetv/ui/screens/list/VideoListScreen.kt

@@ -74,9 +74,14 @@ fun VideoListScreen(
     ),
 ) {
     val items = viewModel.items
-    // per-item focus requesters so BACK from the player can refocus the opened card
+    // per-item focus requesters so BACK from the player can refocus the opened card.
-    val requesters = androidx.compose.runtime.remember(items.size, epoch) {
+    // A plain HashMap, like the rows: this is/was written during composition of
-        androidx.compose.runtime.mutableStateMapOf<Int, FocusRequester>()
+    // every card, and a snapshot map here re-invalidated the reading scopes on
+    // each write — the exact recompose loop behind the old 2fps scroll (5ea1ca5).
+    // Keyed only on [epoch]: on items.size the map was dropped wholesale, and
+    // already-composed cards didn't re-populate it until their next recompose.
+    val requesters = androidx.compose.runtime.remember(epoch) {
+        HashMap<Int, FocusRequester>()
     }
     // Scroll position survives destination switches (per source), not just player exits.
     val gridState = androidx.compose.runtime.saveable.rememberSaveable(