[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>
         </>
     );

client/ui/frontend/src/screens/Networks.tsx

@@ -55,7 +55,7 @@ export default function Networks() {
 
   const overlapping = useMemo(() => filterOverlapping(routes), [routes]);
   const exitNodes = useMemo(
-    () => routes.filter((r) => r.range === "0.0.0.0/0" || r.range === "::/0"),
+    () => routes.filter((r) => isDefaultRoute(r.range)),
     [routes],
   );
 
@@ -146,6 +146,15 @@ function NetworkList({
   );
 }
 
+// range is the merged display string from the daemon, e.g. "0.0.0.0/0",
+// "::/0", or "0.0.0.0/0, ::/0" when a v4 exit node has a paired v6 entry.
+function isDefaultRoute(range: string): boolean {
+  return range.split(",").some((part) => {
+    const trimmed = part.trim();
+    return trimmed === "0.0.0.0/0" || trimmed === "::/0";
+  });
+}
+
 function filterOverlapping(routes: Network[]): Network[] {
   const byRange = new Map<string, Network[]>();
   for (const r of routes) {