Detect interfaces per platform; add macOS and single-interface support (closes #2)

Linux runs exactly as before when both bridge interfaces exist. When they do
not, the lone default-route interface is monitored, and a single interface now
draws a single UI pane instead of an empty second one.

macOS is newly supported: a running VPN tunnel (utun) is monitored as the
primary pane alongside the physical default-route interface, or the physical
interface alone when no VPN is up.

Detection lives in internal/netdetect: interface/route data types, pure
selection logic keyed on OS name, and route parsers, all unit-tested on Linux
for both platforms. Only the real route query and the per-platform TCP dial
binding (source address on Linux, IP_BOUND_IF on macOS) are build-tagged. Ping
argument construction is a pure, OS-keyed function. NewMonitor now takes a list
of interfaces.

Model: opus-4-8

go.mod

@@ -6,6 +6,7 @@ require (
 	github.com/gdamore/tcell/v2 v2.8.1
 	github.com/spf13/cobra v1.8.0
 	github.com/spf13/viper v1.18.2
+	golang.org/x/sys v0.29.0
 )
 
 require (
@@ -29,7 +30,6 @@ require (
 	go.uber.org/atomic v1.9.0 // indirect
 	go.uber.org/multierr v1.9.0 // indirect
 	golang.org/x/exp v0.0.0-20230905200255-921286631fa9 // indirect
-	golang.org/x/sys v0.29.0 // indirect
 	golang.org/x/term v0.28.0 // indirect
 	golang.org/x/text v0.21.0 // indirect
 	gopkg.in/ini.v1 v1.67.0 // indirect