fix: wrap tabBarMinimizeBehavior in availability check for iOS 17 compat

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ios/Platform/Platform/ContentView.swift

@@ -43,7 +43,7 @@ struct MainTabView: View {
                     .tag(2)
             }
             .tint(Color.accentWarm)
-            .tabBarMinimizeBehavior(.onScrollDown)
+            .modifier(TabBarMinimizeModifier())
 
             // Floating buttons
             VStack {
@@ -150,3 +150,15 @@ struct AssistantSheetView: View {
         }
     }
 }
+
+// MARK: - Tab Bar Minimize (iOS 26+)
+
+struct TabBarMinimizeModifier: ViewModifier {
+    func body(content: Content) -> some View {
+        if #available(iOS 26.0, *) {
+            content.tabBarMinimizeBehavior(.onScrollDown)
+        } else {
+            content
+        }
+    }
+}