fix: [IPv6 Port Service] back button incorrectly navigates to Menu instead of Advanced Settings

[PeterJhongLinksys]

Description

On the IPv6 Port Service page, tapping the back button navigates to the Menu page instead of returning to Advanced Settings.

Root Cause

The backFallback in lib/page/ipv6_port_service/views/usp_ipv6_port_service_view.dart:29 is set to RouteNamed.uspAdvancedSettings, and the route is correctly defined as a child of uspAdvancedSettings in lib/route/route_usp_dashboard.dart:137. Despite this configuration, the back navigation does not resolve to the parent route as expected and instead falls through to the Menu page.

Expected Behavior

Tapping the back button on the IPv6 Port Service page should return the user to the Advanced Settings page.

[PeterJhongLinksys]

[Root Cause]
The IPv6 Port Service view used backFallback: RouteNamed.uspAdvancedSettings, but since the route is nested under Advanced Settings in the ShellRoute, canPop() returns true and pop() navigates to the parent route (Advanced Settings) — the backFallback was never reached. The only entry point to IPv6 Port Service is the Firewall page.

[Solution]
Replaced backFallback with onBackTap: () => context.goNamed(RouteNamed.uspFirewall) to explicitly navigate back to Firewall, bypassing the canPop()/pop() parent route behavior.

Commit: de3bdf0