[management] Add stable domain resolution for combined server (#5515)

The combined server was using the hostname from exposedAddress for both
singleAccountModeDomain and dnsDomain, causing fresh installs to get
the wrong domain and existing installs to break if the config changed.
 Add resolveDomains() to BaseServer that reads domain from the store:
  - Fresh install (0 accounts): uses "netbird.selfhosted" default
  - Existing install: reads persisted domain from the account in DB
  - Store errors: falls back to default safely

The combined server opts in via AutoResolveDomains flag, while the
 standalone management server is unaffected.

combined/cmd/root.go

@@ -493,9 +493,6 @@ func handleTLSConfig(cfg *CombinedConfig) (*tls.Config, bool, error) {
 func createManagementServer(cfg *CombinedConfig, mgmtConfig *nbconfig.Config) (*mgmtServer.BaseServer, error) {
 	mgmt := cfg.Management
 
-	dnsDomain := mgmt.DnsDomain
-	singleAccModeDomain := dnsDomain
-
 	// Extract port from listen address
 	_, portStr, err := net.SplitHostPort(cfg.Server.ListenAddress)
 	if err != nil {
@@ -507,8 +504,9 @@ func createManagementServer(cfg *CombinedConfig, mgmtConfig *nbconfig.Config) (*
 	mgmtSrv := mgmtServer.NewServer(
 		&mgmtServer.Config{
 			NbConfig:                mgmtConfig,
-			DNSDomain:               dnsDomain,
-			MgmtSingleAccModeDomain: singleAccModeDomain,
+			DNSDomain:               "",
+			MgmtSingleAccModeDomain: "",
+			AutoResolveDomains:      true,
 			MgmtPort:                mgmtPort,
 			MgmtMetricsPort:         cfg.Server.MetricsPort,
 			DisableMetrics:          mgmt.DisableAnonymousMetrics,