Move window controls into the title bar

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/app.rs

@@ -193,6 +193,7 @@ pub(crate) enum Message {
     ToggleArchivedSessions(i32),
     ShowChat,
     ShowStats,
+    ToggleSidebar,
     MetricsTick,
 }
 
@@ -403,6 +404,15 @@ impl App {
             }
             Message::ShowChat => self.detail_tab = DetailTab::Chat,
             Message::ShowStats => self.detail_tab = DetailTab::Stats,
+            Message::ToggleSidebar => {
+                let collapsed = !self.preferences.sidebar_collapsed;
+                self.preferences.sidebar_collapsed = collapsed;
+                if let Some(database) = self.database.as_mut()
+                    && let Err(error) = database.set_sidebar_collapsed(collapsed)
+                {
+                    self.error = Some(error);
+                }
+            }
             Message::MetricsTick => self.sample_metrics(),
             Message::PreferenceModelChanged(model) => {
                 self.preference_draft.model = model;