This change adds Linux support for Desktop VPN by aligning Linux
behavior with the existing Windows daemon implementation and adding a
Linux networking stack implementation.
### What changed
- Consolidated the daemon command implementation into a shared file:
- `cli/vpndaemon_windows_linux.go` (`//go:build windows || linux`)
- Consolidated daemon tests into a shared file:
- `cli/vpndaemon_windows_linux_test.go` (`//go:build windows || linux`)
- Removed Linux-only duplicate daemon files:
- `cli/vpndaemon_linux.go`
- `cli/vpndaemon_linux_test.go`
- Removed unsupported-platform stubs per current supported OS targets:
- `cli/vpndaemon_other.go`
- `vpn/tun.go`
- Kept Linux networking stack implementation in:
- `vpn/tun_linux.go`
### Notes
- Linux now uses the same `rpc-read-handle` / `rpc-write-handle` flags
and env vars as Windows.
- The daemon logs to stderr (via CLI logger sinks), and does not forward
logs over the RPC pipe.
Michael Suchacz