[client] Add IPv6 dual-stack UI support for Android, iOS, and desktop

[lixmal]

Describe your changes

Merge v4/v6 exit node route pairs into a single UI entry across all platforms (Android, iOS, desktop, daemon CLI). When the management server creates a paired exit node with -v6 suffix, the client now displays one combined entry (0.0.0.0/0, ::/0) with a single toggle that selects/deselects both.

• Merge paired v4+v6 exit node routes into one display entry on Android, iOS, server daemon, and desktop UI
• Add ExpandV6ExitPairs in the shared route package so select/deselect auto-includes the paired v6 route
• Add IsV4DefaultRoute/IsV6DefaultRoute helpers using typed netip.Prefix comparisons
• Show the peer actively routing traffic for HA route groups instead of always picking the first peer
• Include IPv6 address in onAddressChanged notifications for dual-stack display on the home screen
• Fix iOS nil-check ordering in GetRoutesSelectionDetails

Stacked on #5708.

Issue ticket number and link

Stack

• [management, client] Add proto fields for IPv6 overlay and compact prefix encoding #5631
  • [client] Add IPv6 overlay address support to WireGuard interface and engine #5675
    • [client] Add IPv6 reverse DNS and host configurator support #5686
      • [client] Add IPv6 support to SSH server, client config, and netflow logger #5687
        • [client] Add IPv6 support to ACL manager, USP filter, and forwarder #5688
          • [client] Add IPv6 routing, fake IPs, and DNS bind selection #5706
            • [client] Add dual-stack nftables manager with IPv6 table support #5707
              • [client] Add dual-stack iptables manager with ip6tables support #5708
                • [client] Add IPv6 dual-stack UI support for Android, iOS, and desktop #5738 👈

Checklist

• Is it a bug fix
• Is a typo/documentation fix
• Is a feature enhancement
• It is a refactor
• Created tests that fail without the change (if possible)

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Documentation

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud