feat(tui): add monitor groups with collapse/expand and tree view

Groups act as visual organizers in the sites table. Monitors can be
assigned to a parent group via the form. Group rows show aggregated
worst-child status, children render with tree chars (├/└), and Space
toggles collapse/expand. Group form hides irrelevant connection and
advanced sections.

internal/monitor/monitor.go

@@ -243,7 +243,7 @@ func checkByID(id int) {
 	case "dns":
 		checkDNS(site)
 	case "group":
-		// groups don't perform checks
+		checkGroup(site)
 	}
 }
 
@@ -437,6 +437,44 @@ func checkPort(site models.Site) {
 	handleStatusChange(updatedSite, "UP", 0, latency)
 }
 
+func checkGroup(site models.Site) {
+	Mutex.RLock()
+	status := "UP"
+	hasChildren := false
+	allPaused := true
+	for _, child := range LiveState {
+		if child.ParentID != site.ID || child.Type == "group" {
+			continue
+		}
+		hasChildren = true
+		if !child.Paused {
+			allPaused = false
+		}
+		if child.Paused {
+			continue
+		}
+		if child.Status == "DOWN" || child.Status == "SSL EXP" {
+			status = "DOWN"
+		} else if child.Status == "PENDING" && status != "DOWN" {
+			status = "PENDING"
+		}
+	}
+	Mutex.RUnlock()
+
+	if !hasChildren {
+		status = "PENDING"
+	}
+
+	Mutex.Lock()
+	s := LiveState[site.ID]
+	s.Status = status
+	if hasChildren && allPaused {
+		s.Paused = true
+	}
+	LiveState[site.ID] = s
+	Mutex.Unlock()
+}
+
 func checkDNS(site models.Site) {
 	host := site.Hostname
 	if host == "" {