[client] Sweep network-bound connections when the OS switches networks

On a network switch (e.g. cellular to WiFi) the management, signal and
relay sockets stay bound to the old network and look alive until the OS
tears them down — measured at 5 seconds of dead air on Android, while
the UI kept claiming Connected. The Android client papered over this
with a full engine restart, paying for it with a torn-down TUN device
and discarded peer state.

Introduce client/netsweep: connections register on dial and deregister
on close, and a sweep closes everything registered while aborting
in-flight dials through sweep-cancellable dial contexts. The aborted
dials matter: a relay dial started on the dying network would otherwise
hold the reconnect loop hostage for the QUIC handshake timeout. After a
sweep every failure surfaces as an ordinary read/write error and the
existing retry loops redial immediately on the new network.

The sweeper reaches the three long-lived connections through the same
options that carry the netstate gate: a gRPC dial option wraps the
management and signal transports (reconnects included), and the relay
client wraps its connection in one place for the picker, the guard and
foreign relays alike. Everything is nil-safe; platforms that inject no
sweeper are untouched.

Mobile clients expose the sweep as NotifyNetworkChange. Measured on
Android against the engine restart it replaces: recovery in 1.6s
instead of 3.2s, no Disconnected flash, and the TUN device, WireGuard
config and peer state survive.

shared/relay/client/client.go

@@ -14,6 +14,7 @@ import (
 
 	log "github.com/sirupsen/logrus"
 
+	"github.com/netbirdio/netbird/client/netsweep"
 	auth "github.com/netbirdio/netbird/shared/relay/auth/hmac"
 	"github.com/netbirdio/netbird/shared/relay/client/dialer"
 	netErr "github.com/netbirdio/netbird/shared/relay/client/dialer/net"
@@ -184,6 +185,10 @@ type Client struct {
 	// datagram-sized transport is avoided on subsequent connects. Shared via
 	// the manager.
 	transportFallback *transportFallback
+
+	// sweeper cuts the relay connection on network change; the read loop
+	// reports the disconnect and the guard reconnects. Shared via the manager.
+	sweeper *netsweep.Sweeper
 	// datagramFallbackTriggered guards a single fallback per connection so a
 	// burst of oversized datagrams triggers one reconnect, not many.
 	datagramFallbackTriggered atomic.Bool
@@ -393,6 +398,11 @@ func (c *Client) Close() error {
 }
 
 func (c *Client) connect(ctx context.Context) (*RelayAddr, error) {
+	// A sweep cancels this context, so a dial started on the old network
+	// aborts instead of waiting out its handshake timeout.
+	ctx, releaseDial := c.sweeper.WrapDialContext(ctx)
+	defer releaseDial()
+
 	mode := transportModeFromEnv()
 	dialers := c.getDialers(mode)
 
@@ -417,6 +427,7 @@ func (c *Client) connect(ctx context.Context) (*RelayAddr, error) {
 			return nil, fmt.Errorf("dial via FQDN: %w", err)
 		}
 	}
+	conn = c.sweeper.WrapConn(conn)
 	c.relayConn = conn
 	c.datagramFallbackTriggered.Store(false)
 	if tc, ok := conn.(transportConn); ok {

shared/relay/client/manager.go

@@ -13,6 +13,7 @@ import (
 	log "github.com/sirupsen/logrus"
 
 	"github.com/netbirdio/netbird/client/netstate"
+	"github.com/netbirdio/netbird/client/netsweep"
 	relayAuth "github.com/netbirdio/netbird/shared/relay/auth/hmac"
 )
 
@@ -72,6 +73,11 @@ func WithNetworkState(netState *netstate.State) ManagerOption {
 	return func(m *Manager) { m.netState = netState }
 }
 
+// WithSweeper injects the network change sweeper.
+func WithSweeper(sweeper *netsweep.Sweeper) ManagerOption {
+	return func(m *Manager) { m.sweeper = sweeper }
+}
+
 // Manager is a manager for the relay client instances. It establishes one persistent connection to the given relay URL
 // and automatically reconnect to them in case disconnection.
 // The manager also manage temporary relay connection. If a client wants to communicate with a client on a
@@ -100,6 +106,7 @@ type Manager struct {
 	mtu                uint16
 	maxBackoffInterval time.Duration
 	netState           *netstate.State
+	sweeper            *netsweep.Sweeper
 
 	cleanupInterval      time.Duration
 	keepUnusedServerTime time.Duration
@@ -136,6 +143,7 @@ func NewManager(ctx context.Context, serverURLs []string, peerID string, mtu uin
 	for _, opt := range opts {
 		opt(m)
 	}
+	m.serverPicker.Sweeper = m.sweeper
 	m.serverPicker.ServerURLs.Store(serverURLs)
 	m.reconnectGuard = NewGuard(m.serverPicker, m.maxBackoffInterval, m.netState)
 	return m
@@ -362,6 +370,7 @@ func (m *Manager) openConnVia(ctx context.Context, serverAddress, peerKey string
 
 	relayClient := NewClientWithServerIP(serverAddress, serverIP, m.tokenStore, m.peerID, m.mtu)
 	relayClient.SetTransportFallback(m.transportFallback)
+	relayClient.sweeper = m.sweeper
 	err := relayClient.Connect(m.ctx)
 	if err != nil {
 		rt.Lock()

shared/relay/client/picker.go

@@ -9,6 +9,7 @@ import (
 
 	log "github.com/sirupsen/logrus"
 
+	"github.com/netbirdio/netbird/client/netsweep"
 	auth "github.com/netbirdio/netbird/shared/relay/auth/hmac"
 )
 
@@ -30,6 +31,7 @@ type ServerPicker struct {
 	MTU               uint16
 	ConnectionTimeout time.Duration
 	TransportFallback *transportFallback
+	Sweeper           *netsweep.Sweeper
 }
 
 func (sp *ServerPicker) PickServer(parentCtx context.Context) (*Client, error) {
@@ -73,6 +75,7 @@ func (sp *ServerPicker) startConnection(ctx context.Context, resultChan chan con
 	log.Infof("try to connecting to relay server: %s", url)
 	relayClient := NewClient(url, sp.TokenStore, sp.PeerID, sp.MTU)
 	relayClient.SetTransportFallback(sp.TransportFallback)
+	relayClient.sweeper = sp.Sweeper
 	err := relayClient.Connect(ctx)
 	resultChan <- connResult{
 		RelayClient: relayClient,