fix(desktop): Messages divider blends into both panes

Split the 12dp draggable divider into two 6dp halves: left half takes
surfaceContainer (matching the conversation list pane's fill), right
half takes surface (matching the chat pane's fill). The divider now
reads as the shared edge between the two panes instead of a solid
contrasting stripe cutting down the middle.

https://claude.ai/code/session_01NufduPfZvYQVYwLkbCjCUo

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/ui/chats/DesktopMessagesScreen.kt

@@ -33,7 +33,6 @@ import androidx.compose.foundation.layout.width
 import androidx.compose.material3.Icon
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Text
-import androidx.compose.material3.VerticalDivider
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.collectAsState
 import androidx.compose.runtime.getValue
@@ -381,12 +380,15 @@ private fun EmptyConversationState() {
 
 /**
  * Draggable vertical divider between the conversation list pane and the chat
- * pane. 12dp wide hit area; cursor flips to the horizontal resize arrow on
- * hover.
+ * pane. The hit area is split in half horizontally: the left half takes the
+ * conversation list's surfaceContainer fill, the right half takes the chat
+ * pane's surface fill, so the divider reads as a continuation of the two panes
+ * instead of a single contrasting stripe. Cursor flips to the horizontal
+ * resize arrow on hover.
  */
 @Composable
 private fun MessagesDraggableDivider(onDrag: (Float) -> Unit) {
-    Box(
+    Row(
         modifier =
             Modifier
                 .width(12.dp)
@@ -398,8 +400,20 @@ private fun MessagesDraggableDivider(onDrag: (Float) -> Unit) {
                         onDrag(dragAmount.x)
                     }
                 },
-        contentAlignment = Alignment.Center,
     ) {
-        VerticalDivider(color = MaterialTheme.colorScheme.outlineVariant)
+        Box(
+            modifier =
+                Modifier
+                    .width(6.dp)
+                    .fillMaxHeight()
+                    .background(MaterialTheme.colorScheme.surfaceContainer),
+        )
+        Box(
+            modifier =
+                Modifier
+                    .width(6.dp)
+                    .fillMaxHeight()
+                    .background(MaterialTheme.colorScheme.surface),
+        )
     }
 }