Add NetBird as optional VPN service

[n0pashkov]

Summary

• Adds NetBird to the Install → Service menu alongside Tailscale and NordVPN
• New omarchy-install-netbird script installs the daemon (netbird AUR), enables it via systemd, installs the TUI client (netbird-tui AUR), and registers it with omarchy-tui
• Adds official NetBird icon (applications/icons/NetBird.png)
• No default install — user-initiated only via the menu

Test plan

• Open Install → Service menu and confirm 󰌘 NetBird [AUR] appears after NordVPN
• Select NetBird and verify the installer script runs in a floating terminal
• Confirm netbird daemon is enabled and netbird-tui is accessible after install
• Run sudo netbird up to verify VPN connectivity