[management] Add dashboard_features account setting for per-section visibility

Introduce a nullable dashboard_features object on account settings, serialized
to a single JSON column so new dashboard sections can be added without schema
changes. Starts with agent_network (show the Agent Network menu for an account
without the deployment flag). Wires the API handler mapping, the pgx GetAccount
loader, and adds store round-trip and handler tests.

management/server/types/settings.go

@@ -80,6 +80,11 @@ type Settings struct {
 	// Set for accounts created via netbird.ai signups; users can disable it later.
 	AgentNetworkOnly bool `gorm:"default:false"`
 
+	// DashboardFeatures holds per-account dashboard section visibility overrides.
+	// It serializes to a single JSON column so new sections can be added without
+	// a schema change.
+	DashboardFeatures *DashboardFeatures `gorm:"serializer:json"`
+
 	// EmbeddedIdpEnabled indicates if the embedded identity provider is enabled.
 	// This is a runtime-only field, not stored in the database.
 	EmbeddedIdpEnabled bool `gorm:"-"`
@@ -126,9 +131,31 @@ func (s *Settings) Copy() *Settings {
 	if s.Extra != nil {
 		settings.Extra = s.Extra.Copy()
 	}
+	if s.DashboardFeatures != nil {
+		settings.DashboardFeatures = s.DashboardFeatures.Copy()
+	}
 	return settings
 }
 
+// DashboardFeatures holds per-account dashboard section visibility overrides.
+// Nil fields are unset and follow the default dashboard behavior; an explicit
+// value forces that section shown or hidden for the account.
+type DashboardFeatures struct {
+	// AgentNetwork, when set, forces the Agent Network menu shown (true) or
+	// hidden (false) regardless of the deployment feature flag.
+	AgentNetwork *bool `json:"agent_network,omitempty"`
+}
+
+// Copy returns a deep copy of the DashboardFeatures struct.
+func (d *DashboardFeatures) Copy() *DashboardFeatures {
+	c := &DashboardFeatures{}
+	if d.AgentNetwork != nil {
+		v := *d.AgentNetwork
+		c.AgentNetwork = &v
+	}
+	return c
+}
+
 type ExtraSettings struct {
 	// PeerApprovalEnabled enables or disables the need for peers bo be approved by an administrator
 	PeerApprovalEnabled bool