Summary

Adds Linux support for the Desktop VPN daemon by sharing the Windows/Linux CLI implementation and wiring up a Linux networking stack, while removing unsupported-platform stubs. Also hardens the shared daemon tests so in-process runs do not double-close RPC pipe handles.

Changes

• Shared Windows/Linux daemon implementation and tests in OS-agnostic files (cli/vpndaemon_windows_linux_shared*.go) with //go:build windows || linux.
• Added Linux networking stack implementation in vpn/tun_linux.go (TUN + router + DNS configurator).
• Removed unsupported-platform stubs (cli/vpndaemon_other.go, vpn/tun.go).
• Adjusted shared daemon tests to duplicate RPC handles (Linux unix.Dup, Windows syscall.DuplicateHandle) before passing them to the in-process daemon to avoid FD/handle reuse issues.

Testing

• go test -tags=testsmallbatch ./cli -run 'TestVPNDaemonRun|TestOpenApp' -count=1
• go test -race -tags=testsmallbatch ./cli -run 'TestVPNDaemonRun|TestOpenApp' -count=1
• go test -tags=testsmallbatch ./cli -count=1
• go test -race -tags=testsmallbatch ./cli -count=1
• make lint

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Addressed Dean's review feedback in 79888291e:

• Shared Windows/Linux daemon implementation in cli/vpndaemon_windows_linux.go with //go:build windows || linux.
• Shared Windows/Linux tests in cli/vpndaemon_windows_linux_test.go.
• Removed cli/vpndaemon_linux.go, cli/vpndaemon_linux_test.go, and unsupported-platform stubs (cli/vpndaemon_other.go, vpn/tun.go).
• Kept stderr-based logging (no tunnel-pipe logging) and adjusted the handshake test path for race-safety.

Validation run locally:

• go test ./cli -run TestVPNDaemonRun -count=1
• go test -race ./cli -run TestVPNDaemonRun -count=1
• go test ./vpn -count=1
• go build ./cli/...
• go build ./vpn/...
• make lint/go