Session: add a persistent chrome-toggle handle at top center

Three-finger tap still works as a power-user shortcut, but now there's a
small glass chevron pill at the top center that flips between chevron.up
(hide toolbar) and chevron.down (show toolbar) based on chrome state.
Discoverable and reachable from one hand.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Packages/VNCUI/Sources/VNCUI/Session/SessionView.swift

@@ -66,6 +66,13 @@ public struct SessionView: View {
                     overlayChrome(controller: controller)
                         .transition(.opacity.combined(with: .move(edge: .top)))
                 }
+
+                // Persistent pull-tab so the chrome is always reachable
+                // (three-finger tap is the power-user shortcut).
+                VStack {
+                    chromeToggleHandle
+                    Spacer()
+                }
             } else {
                 ProgressView("Preparing session…")
                     .tint(.white)
@@ -95,6 +102,24 @@ public struct SessionView: View {
         }
     }
 
+    private var chromeToggleHandle: some View {
+        Button {
+            withAnimation(.snappy(duration: 0.22)) {
+                chromeVisible.toggle()
+            }
+        } label: {
+            Image(systemName: chromeVisible ? "chevron.up" : "chevron.down")
+                .font(.caption.weight(.bold))
+                .foregroundStyle(.white)
+                .frame(width: 44, height: 20)
+                .contentShape(Rectangle())
+        }
+        .buttonStyle(.plain)
+        .glassSurface(in: Capsule())
+        .accessibilityLabel(chromeVisible ? "Hide toolbar" : "Show toolbar")
+        .padding(.top, chromeVisible ? 4 : 2)
+    }
+
     @ViewBuilder
     private func overlayChrome(controller: SessionController) -> some View {
         VStack(spacing: 8) {