fix(ui): remove duplicate back button from SinglePaneLayout

Overlay screens (UserProfileScreen, ThreadScreen) already render their
own back+title header. Remove the redundant SinglePaneHeader bar.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/ui/deck/SinglePaneLayout.kt

@@ -20,20 +20,15 @@
  */
 package com.vitorpamplona.amethyst.desktop.ui.deck
 
-import androidx.compose.foundation.background
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.Row
-import androidx.compose.foundation.layout.Spacer
 import androidx.compose.foundation.layout.fillMaxHeight
 import androidx.compose.foundation.layout.fillMaxSize
-import androidx.compose.foundation.layout.fillMaxWidth
-import androidx.compose.foundation.layout.height
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.size
 import androidx.compose.foundation.layout.width
 import androidx.compose.material.icons.Icons
-import androidx.compose.material.icons.automirrored.filled.ArrowBack
 import androidx.compose.material.icons.automirrored.filled.Article
 import androidx.compose.material.icons.filled.Bookmark
 import androidx.compose.material.icons.filled.Email
@@ -43,9 +38,7 @@ import androidx.compose.material.icons.filled.Notifications
 import androidx.compose.material.icons.filled.Person
 import androidx.compose.material.icons.filled.Search
 import androidx.compose.material.icons.filled.Settings
-import androidx.compose.material3.HorizontalDivider
 import androidx.compose.material3.Icon
-import androidx.compose.material3.IconButton
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.NavigationRail
 import androidx.compose.material3.NavigationRailItem
@@ -57,7 +50,6 @@ import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
-import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.vector.ImageVector
 import androidx.compose.ui.text.style.TextOverflow
@@ -144,20 +136,6 @@ fun SinglePaneLayout(
         VerticalDivider()
 
         Column(modifier = Modifier.weight(1f).fillMaxHeight()) {
-            // Show header with back button when navigated into overlay
-            if (navStack.isNotEmpty()) {
-                SinglePaneHeader(
-                    title =
-                        when (currentOverlay) {
-                            is DesktopScreen.UserProfile -> "Profile"
-                            is DesktopScreen.Thread -> "Thread"
-                            else -> currentColumnType.title()
-                        },
-                    onBack = { navState.pop() },
-                )
-                HorizontalDivider()
-            }
-
             Box(
                 modifier = Modifier.fillMaxSize().padding(12.dp),
             ) {
@@ -197,37 +175,3 @@ fun SinglePaneLayout(
         }
     }
 }
-
-@Composable
-private fun SinglePaneHeader(
-    title: String,
-    onBack: () -> Unit,
-    modifier: Modifier = Modifier,
-) {
-    Row(
-        modifier =
-            modifier
-                .fillMaxWidth()
-                .height(40.dp)
-                .background(MaterialTheme.colorScheme.surfaceVariant.copy(alpha = 0.5f))
-                .padding(horizontal = 8.dp),
-        verticalAlignment = Alignment.CenterVertically,
-    ) {
-        IconButton(onClick = onBack, modifier = Modifier.size(28.dp)) {
-            Icon(
-                Icons.AutoMirrored.Filled.ArrowBack,
-                contentDescription = "Back",
-                modifier = Modifier.size(16.dp),
-                tint = MaterialTheme.colorScheme.onSurfaceVariant,
-            )
-        }
-        Spacer(Modifier.width(8.dp))
-        Text(
-            text = title,
-            style = MaterialTheme.typography.labelLarge,
-            color = MaterialTheme.colorScheme.onSurfaceVariant,
-            maxLines = 1,
-            overflow = TextOverflow.Ellipsis,
-        )
-    }
-}