perf: use drawBehind for background color to avoid recomposition

Replace background(backgroundColor.value) with drawBehind { drawRect() }
so the color state is read during the draw phase, not composition.
This prevents NoteComposeLayout from recomposing when the "new item"
background color changes after 5 seconds.

https://claude.ai/code/session_01Fam5VHNaNkKBBafg1gRfFS

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/note/NoteCompose.kt

@@ -51,6 +51,7 @@ import androidx.compose.runtime.remember
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Alignment.Companion.CenterVertically
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.drawBehind
 import androidx.compose.ui.graphics.Brush
 import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.graphics.compositeOver
@@ -638,7 +639,10 @@ fun InnerNoteWithReactions(
     val showSecondRow = isNotRepost && notBoostedNorQuote && accountViewModel.settings.isCompleteUIMode()
 
     NoteComposeLayout(
-        modifier = clickModifier.background(backgroundColor.value).fillMaxWidth(),
+        modifier =
+            clickModifier
+                .drawBehind { drawRect(backgroundColor.value) }
+                .fillMaxWidth(),
         addPadding = !isBoostedNote,
         showAuthorColumn = notBoostedNorQuote,
         showSecondRow = showSecondRow,