refactor(tui): status icons, clean STATUS column, relative time

- STATUS column shows icon + clean state only (▲ UP, ▼ DOWN, ◆ LATE,
  ◆ STALE, ◇ PAUSED, ◼ MAINT, ○ PENDING). Error classification
  (DNS/TLS/TMO) removed from STATUS — stays in NAME inline hint.
- Detail panel Last Check shows relative time ("12s ago") instead of
  absolute timestamp.
- Extract shared fmtTimeAgo() to format.go, consolidate duplicate
  formatters in tab_alerts.go and tab_nodes.go.

internal/tui/tab_nodes.go

@@ -1,7 +1,6 @@
 package tui
 
 import (
-	"fmt"
 	"time"
 )
 
@@ -66,15 +65,5 @@ func fmtNodeStatus(lastSeen time.Time) string {
 }
 
 func fmtNodeLastSeen(t time.Time) string {
-	if t.IsZero() {
-		return subtleStyle.Render("never")
-	}
-	ago := time.Since(t)
-	if ago < time.Minute {
-		return fmt.Sprintf("%ds ago", int(ago.Seconds()))
-	}
-	if ago < time.Hour {
-		return fmt.Sprintf("%dm ago", int(ago.Minutes()))
-	}
-	return fmt.Sprintf("%dh ago", int(ago.Hours()))
+	return fmtTimeAgo(t)
 }