fix(desktop): extend content under macOS title bar, not above it

Previous approach reserved a 28dp strip across the whole top of the
window for traffic lights — wasting the area right of the lights that
Slack / Notion / Safari-style apps reuse for app content.

Now the sidebar / NavigationRail extends to the top of the window
(their surfaceContainer covers the traffic lights on the left), and
the main content area (deck columns / single-pane content) also
extends to the top since it's to the right of the lights.

- DeckSidebar: top padding now 8dp + titleBarInsetTop so icons clear
  the traffic lights.
- SinglePaneLayout's NavigationRail: uses its `header` slot to hold a
  Spacer of titleBarInsetTop, same effect.
- Removed the full-width title bar strip from MainContent.
- SinglePaneLayout NavigationRail container color switched from
  surfaceVariant to surfaceContainer to match the deck sidebar and
  the Material 3 sidebar convention.

https://claude.ai/code/session_01NufduPfZvYQVYwLkbCjCUo

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/Main.kt

@@ -1117,19 +1117,6 @@ fun MainContent(
     ) {
         Box(Modifier.fillMaxSize()) {
             Column(Modifier.fillMaxSize()) {
-                // macOS: reserve a title bar strip so deck/sidebar content doesn't
-                // underlap the traffic lights. The strip is colored to match the
-                // sidebar so the whole top edge reads as one continuous toolbar.
-                if (!isImmersive &&
-                    com.vitorpamplona.amethyst.desktop.platform.PlatformInfo.isMacOS
-                ) {
-                    Box(
-                        Modifier
-                            .fillMaxWidth()
-                            .height(com.vitorpamplona.amethyst.desktop.platform.titleBarInsetTop)
-                            .background(MaterialTheme.colorScheme.surfaceContainer),
-                    )
-                }
                 Row(Modifier.fillMaxSize().weight(1f)) {
                     when (layoutMode) {
                         LayoutMode.SINGLE_PANE -> {

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

@@ -43,6 +43,7 @@ import com.vitorpamplona.amethyst.commons.icons.symbols.Icon
 import com.vitorpamplona.amethyst.commons.icons.symbols.MaterialSymbols
 import com.vitorpamplona.amethyst.commons.tor.TorServiceStatus
 import com.vitorpamplona.amethyst.commons.ui.components.BunkerHeartbeatIndicator
+import com.vitorpamplona.amethyst.desktop.platform.titleBarInsetTop
 import com.vitorpamplona.amethyst.desktop.ui.tor.TorStatusIndicator
 
 @Composable
@@ -60,7 +61,7 @@ fun DeckSidebar(
                 .width(56.dp)
                 .fillMaxHeight()
                 .background(MaterialTheme.colorScheme.surfaceContainer)
-                .padding(vertical = 8.dp),
+                .padding(top = 8.dp + titleBarInsetTop, bottom = 8.dp),
         horizontalAlignment = Alignment.CenterHorizontally,
         verticalArrangement = Arrangement.Top,
     ) {

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

@@ -26,6 +26,7 @@ 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.height
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.size
 import androidx.compose.foundation.layout.width
@@ -53,6 +54,7 @@ import com.vitorpamplona.amethyst.desktop.account.AccountState
 import com.vitorpamplona.amethyst.desktop.cache.DesktopLocalCache
 import com.vitorpamplona.amethyst.desktop.network.DesktopRelayConnectionManager
 import com.vitorpamplona.amethyst.desktop.network.Nip11Fetcher
+import com.vitorpamplona.amethyst.desktop.platform.titleBarInsetTop
 import com.vitorpamplona.amethyst.desktop.service.highlights.DesktopHighlightStore
 import com.vitorpamplona.amethyst.desktop.subscriptions.DesktopRelaySubscriptionsCoordinator
 import com.vitorpamplona.amethyst.desktop.ui.ZapFeedback
@@ -98,7 +100,9 @@ fun SinglePaneLayout(
         if (!isImmersive) {
             NavigationRail(
                 modifier = Modifier.width(80.dp).fillMaxHeight(),
-                containerColor = MaterialTheme.colorScheme.surfaceVariant,
+                containerColor = MaterialTheme.colorScheme.surfaceContainer,
+                // macOS: push rail items below the traffic lights.
+                header = { Spacer(Modifier.height(titleBarInsetTop)) },
             ) {
                 val pinnedScreens by pinnedNavBarState.pinnedScreens.collectAsState()
                 pinnedScreens.forEach { screenType ->