routemanager: enforce a single selected exit node

Exit nodes are mutually exclusive, but the RouteSelector stores routes with
default-on semantics, so every available exit node reported as selected at once.

Reconcile exit-node selection on each network map (and on runtime selection):
keep at most one selected — the user's persisted pick, else whatever management
marks for auto-apply (SkipAutoApply=false), else none. Never auto-activate an
exit node the map doesn't request; it stays off until the user picks it.

The server deselects sibling exit nodes when the user activates one (leaving
non-exit routes untouched), and the tray/React exit-node toggle now appends so
activating an exit node no longer wipes network-route selections.

client/server/network_exitnode_test.go

@@ -0,0 +1,26 @@
+package server
+
+import (
+	"net/netip"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/netbirdio/netbird/route"
+)
+
+func TestExitNodeSelectionHelpers(t *testing.T) {
+	routesMap := map[route.NetID][]*route.Route{
+		"exitA": {{Network: netip.MustParsePrefix("0.0.0.0/0")}},
+		"exitB": {{Network: netip.MustParsePrefix("::/0")}},
+		"lan":   {{Network: netip.MustParsePrefix("192.168.0.0/16")}},
+	}
+
+	assert.True(t, requestActivatesExitNode([]route.NetID{"exitA"}, routesMap), "v4 default route is an exit node")
+	assert.True(t, requestActivatesExitNode([]route.NetID{"exitB"}, routesMap), "v6 default route is an exit node")
+	assert.False(t, requestActivatesExitNode([]route.NetID{"lan"}, routesMap), "lan route is not an exit node")
+	assert.False(t, requestActivatesExitNode([]route.NetID{"missing"}, routesMap), "unknown id is not an exit node")
+
+	others := otherExitNodeIDs(routesMap, []route.NetID{"exitB"})
+	assert.ElementsMatch(t, []route.NetID{"exitA"}, others, "only the other exit node is a sibling; the lan route is ignored")
+}