[client] Reconnect management and signal promptly after a network change

Three pieces, each verified on Android across WiFi/cellular switches:

- A sweep-aware backoff wrapper: the first retry after a disconnect
  that follows a recent network-change mark comes after 200ms instead
  of the randomized [0..1.6s] interval. Any other failure keeps the
  unchanged spread, so the clients of a restarted server still scatter
  their reconnects.
- The retry sleep wakes on OS network availability transitions
  (nbgrpc.Retry): a disconnect that precedes the offline flag by a few
  milliseconds no longer sleeps blindly through the recovery - the
  loop parks on the netstate gate and resumes the moment the network
  returns.
- The connection state is re-checked after WaitForStateChange: a dial
  settling in Ready proceeds immediately instead of burning another
  backoff round on an already-usable channel.

Measured after a network switch: management and signal recover in
270-470ms deterministically, down from a 312-1593ms lottery.

client/netsweep/quick_retry.go

@@ -0,0 +1,39 @@
+package netsweep
+
+import (
+	"time"
+
+	"github.com/cenkalti/backoff/v4"
+
+	"github.com/netbirdio/netbird/client/netstate"
+)
+
+const quickRetryDelay = 200 * time.Millisecond
+
+type quickRetryBackoff struct {
+	backoff.BackOff
+	sweeper  *Sweeper
+	netState *netstate.State
+	used     bool
+}
+
+func newQuickRetryBackoff(bo backoff.BackOff, sweeper *Sweeper, netState *netstate.State) *quickRetryBackoff {
+	return &quickRetryBackoff{
+		BackOff:  bo,
+		sweeper:  sweeper,
+		netState: netState,
+	}
+}
+
+func (b *quickRetryBackoff) NextBackOff() time.Duration {
+	if !b.used && b.sweeper.markedRecently() && b.netState.IsOnline() {
+		b.used = true
+		return quickRetryDelay
+	}
+	return b.BackOff.NextBackOff()
+}
+
+func (b *quickRetryBackoff) Reset() {
+	b.used = false
+	b.BackOff.Reset()
+}