Add Linux systemd service support #15

Description

Implement systemd user service support for Linux as part of the platform abstraction effort.

Tasks

1. Create `internal/daemon/service_darwin.go` — extract existing macOS launchctl code
2. Create `internal/daemon/service_linux.go` — systemd unit file generation + `systemctl --user` commands
3. Create `internal/daemon/service.go` — platform-agnostic ServiceManager interface
4. Update `cmd/mnemonic/main.go` install/uninstall to use the new interface
5. Add `runtime.GOOS` guards in start/stop commands

Depends On

• Daemon uses launchctl unconditionally — crashes on Linux #1 (daemon launchctl crash)
• Install/uninstall commands are macOS LaunchAgent only #2 (install/uninstall macOS only)