Improve TCP keepalive and idle timeout for mobile clients

TCP keepalive was configured (SetKeepAlivePeriod) but never actually
enabled (SO_KEEPALIVE) on accepted client connections. Go 1.26's
SetKeepAlivePeriod only sets TCP_KEEPIDLE — it does not call
setsockopt(SO_KEEPALIVE, 1). Without SO_KEEPALIVE the kernel never
sends probe packets, so dead connections from sleeping mobile clients
linger until the idle timeout fires.

Replace SetKeepAlive + SetKeepAlivePeriod with net.KeepAliveConfig
(available since Go 1.24) for explicit per-socket control:

  Idle:     30s   (time before first probe)
  Interval: 10s   (between probes)
  Count:    3     (failed probes to declare dead)

This detects dead connections in ~60s instead of relying on system
defaults (tcp_keepalive_intvl=75s, probes=9 → up to 11 minutes).

Increase the default idle timeout from 1 minute to 5 minutes.
MTProto clients send ping_delay_disconnect every ~60s, which resets
the idle timer. The previous 1-minute default created a race: if a
ping arrived even 1–2 seconds late the relay was killed. A 5-minute
window also survives typical mobile sleep periods (phone idle 2–5 min)
where the NAT mapping is still alive and the connection can resume
without reconnection.

Ref: #132

internal/cli/run_proxy.go

@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"net"
 	"os"
-	"time"
 
 	"github.com/9seconds/mtg/v2/antireplay"
 	"github.com/9seconds/mtg/v2/events"
@@ -263,7 +262,7 @@ func runProxy(conf *config.Config, version string) error { //nolint: funlen
 
 		AllowFallbackOnUnknownDC: conf.AllowFallbackOnUnknownDC.Get(false),
 		TolerateTimeSkewness:     conf.TolerateTimeSkewness.Value,
-		IdleTimeout:              conf.Network.Timeout.Idle.Get(time.Minute),
+		IdleTimeout:              conf.Network.Timeout.Idle.Get(mtglib.DefaultIdleTimeout),
 
 		DoppelGangerURLs:    doppelGangerURLs,
 		DoppelGangerPerRaid: conf.Defense.Doppelganger.Repeats.Get(mtglib.DoppelGangerPerRaid),