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/internal/routeselector/routeselector.go

@@ -124,6 +124,16 @@ func (rs *RouteSelector) IsSelected(routeID route.NetID) bool {
 	return rs.isSelectedLocked(routeID)
 }
 
+// IsDeselectAllActive reports whether the global "deselect all" flag is set,
+// i.e. the user disabled every route. Callers enforcing per-route invariants
+// (e.g. single exit node) should leave the selection untouched when it is.
+func (rs *RouteSelector) IsDeselectAllActive() bool {
+	rs.mu.RLock()
+	defer rs.mu.RUnlock()
+
+	return rs.deselectAll
+}
+
 // FilterSelected removes unselected routes from the provided map.
 func (rs *RouteSelector) FilterSelected(routes route.HAMap) route.HAMap {
 	rs.mu.RLock()