tailscale: Match bare hostnames in preferred_by when search domains exist

MagicDNS resolves single-label names via tailnet search domains, so
route preferred_by should treat them as tailscale domains, matching
official client behavior.

protocol/tailscale/endpoint.go

@@ -98,6 +98,7 @@ type Endpoint struct {
 	routerCfg     *router.Config
 	dnsCfg        *tsDNS.Config
 	routeDomains  common.TypedValue[map[string]bool]
+	searchDomains atomic.Bool
 	routePrefixes atomic.Pointer[netipx.IPSet]
 
 	acceptRoutes               bool
@@ -785,7 +786,11 @@ func (t *Endpoint) PreferredDomain(domain string) bool {
 	if routeDomains == nil {
 		return false
 	}
-	return routeDomains[strings.ToLower(domain)]
+	domain = strings.ToLower(domain)
+	if routeDomains[domain] {
+		return true
+	}
+	return !strings.Contains(domain, ".") && t.searchDomains.Load()
 }
 
 func (t *Endpoint) PreferredAddress(address netip.Addr) bool {
@@ -830,6 +835,7 @@ func (t *Endpoint) onReconfig(cfg *wgcfg.Config, routerCfg *router.Config, dnsCf
 		routeDomains[fqdn.WithoutTrailingDot()] = true
 	}
 	t.routeDomains.Store(routeDomains)
+	t.searchDomains.Store(len(dnsCfg.SearchDomains) > 0)
 
 	var builder netipx.IPSetBuilder
 	for _, peer := range cfg.Peers {