[client/ui] Port IPv6 toggle and paired default-route filter to Wails UI

Brings two main-side PRs' UI behavior across the Fyne→Wails rewrite:

- #5631 (IPv6 overlay support): add "Enable IPv6" row to the polished
  SettingsNetwork tab; the legacy screens/Settings.tsx already had it,
  but modules/settings/SettingsNetwork.tsx (the user-visible Settings
  window) was missing the toggle.
- #6150 (mirror v4 exit selection onto v6 pair): replace the literal
  "0.0.0.0/0" || "::/0" filter in screens/Networks.tsx with an
  isDefaultRoute() helper that handles the daemon's merged-range
  display string (e.g. "0.0.0.0/0, ::/0"), so paired v4/v6 exit
  nodes are classified correctly.

client/ui/frontend/src/modules/settings/SettingsNetwork.tsx

@@ -45,6 +45,12 @@ export function SettingsNetwork() {
                     label={"Enable Server Routes"}
                     helpText={"Advertise this host's local routes to other peers."}
                 />
+                <FancyToggleSwitch
+                    value={!config.disableIpv6}
+                    onChange={(v) => setField("disableIpv6", !v)}
+                    label={"Enable IPv6"}
+                    helpText={"Use IPv6 addressing for the NetBird overlay network."}
+                />
             </SectionGroup>
         </>
     );