[client] Fix iOS DNS upstream routing for deselected exit nodes (#5803)

- Add GetSelectedClientRoutes() to the route manager that filters through FilterSelectedExitNodes, returning only active routes instead of all management routes              
  - Use GetSelectedClientRoutes() in the DNS route checker so deselected exit nodes' 0.0.0.0/0 no longer matches upstream DNS IPs — this prevented the resolver from switching
  away from the utun-bound socket after exit node deselection                                                                                                                   
  - Initialize iOS DNS server with host DNS fallback addresses (1.1.1.1:53, 1.0.0.1:53) and a permanent root zone handler, matching Android's behavior — without this, unmatched
   DNS queries arriving via the 0.0.0.0/0 tunnel route had no handler and were silently dropped

client/internal/routemanager/notifier/notifier_ios.go

@@ -53,7 +53,6 @@ func (n *Notifier) OnNewPrefixes(prefixes []netip.Prefix) {
 	n.currentPrefixes = newNets
 	n.notify()
 }
-
 func (n *Notifier) notify() {
 	n.listenerMux.Lock()
 	defer n.listenerMux.Unlock()