fix(desktop): drop 12dp border around single-pane content

SinglePaneLayout wrapped the main content area in `.padding(12.dp)`,
leaving a visible strip of the window background around every screen.
On macOS with the theming updates this strip became very noticeable:
conversation list, chat content, and the NavigationRail no longer
shared a single edge, and you could see `#F5F5F7` framing the whole
content block.

Native desktop apps don't frame content like that — padding is added
inside cards / lists / dialogs, not around the window content area.
Removing the wrapper padding lets the Messages two-column layout
(surfaceContainer sidebar + surface chat) run edge-to-edge.

https://claude.ai/code/session_01NufduPfZvYQVYwLkbCjCUo

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

@@ -181,9 +181,10 @@ fun SinglePaneLayout(
         }
 
         Column(modifier = Modifier.weight(1f).fillMaxHeight()) {
-            Box(
+            // Content extends to the window edges; individual screens add their
-                modifier = Modifier.fillMaxSize().padding(if (isImmersive) 0.dp else 12.dp),
+            // own internal padding where appropriate (Messages uses full-bleed
-            ) {
+            // panes to match native two-column chat apps).
+            Box(modifier = Modifier.fillMaxSize()) {
                 // Always keep RootContent composed so state (e.g. search results) survives navigation
                 RootContent(
                     columnType = currentColumnType,