feat: add NetBird VPN support

[AndrewMoryakov]

Summary

• Add NetBird as a second VPN option alongside Tailscale, using the official NetBird cloud server
• VPN autostart mutual exclusion via /etc/kvm/vpn preference file — only one VPN runs at a time (NanoKVM Cube has 161MB RAM)
• Autostart toggle added to both Tailscale and NetBird headers
• Init script updates: swap setup, GOMEMLIMIT, OOM protection for NanoKVM-Server

Details

Backend:

• server/service/extensions/netbird/ — CLI wrapper, bundled binary install, service handlers
• server/service/extensions/vpn/ — VPN preference get/set with automatic stop-old/start-new
• server/proto/netbird.go + vpn.go — request/response types
• Routes: install, uninstall, status, login, down, start, stop, restart + VPN preference

Frontend:

• NetBird settings tab mirroring Tailscale's state machine: Install → Run → Login → Device
• Device flow login: Login button → URL opens in new tab → "I have logged in" confirmation
• Device panel: enable/disable switch, device name, IP, version, disconnect button
• Autostart switch on both Tailscale and NetBird headers with confirmation dialog

Init scripts:

• S99netbird — TUN device setup, GOMEMLIMIT=30MiB, PID management
• S95nanokvm — reads /etc/kvm/vpn, starts only selected VPN, swap setup, OOM protection

Test plan

• Install NetBird from Settings → NetBird tab
• Complete device flow login with official NetBird server
• Verify device info (name, IP, version) displays correctly
• Toggle enable/disable switch on device panel
• Disconnect and reconnect
• Toggle autostart between Tailscale and NetBird — verify only one runs
• Reboot NanoKVM — verify correct VPN starts based on preference
• Uninstall NetBird
• Verify Tailscale still works normally with autostart toggle