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
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 are build-tagged. NewMonitor now takes a list of interfaces.

Not verified on a real mac: live netstat parsing, IP_BOUND_IF dialing, Mullvad leak protection.

Model: opus-4-8 (implementation); fable-5-1 (landing commit)

internal/monitor/styles.go

@@ -1,5 +1,3 @@
-//go:build linux
-
 package monitor
 
 import tcell "github.com/gdamore/tcell/v2"