fix(tui): correct delete confirm labels, normalize help text

Delete confirmation dialog used wrong magic numbers for entity kind
mapping — maintenance windows showed as "alert", settings deletions
showed as "monitor". Now uses tab/section constants to match correctly.

Also: consistent [q/Esc] Back across all views, extract detailKeys(),
remove dead maxSections variable, add nodes empty state hint, use
models.Status constants in siteOrder, delete orphaned theme.png.

internal/tui/format.go

@@ -49,13 +49,13 @@ func siteOrder(s models.Site) int {
 		return 3
 	}
 	switch s.Status {
-	case "DOWN", "SSL EXP":
+	case models.StatusDown, models.StatusSSLExp:
 		return 0
-	case "STALE":
+	case models.StatusStale:
 		return 1
-	case "LATE":
+	case models.StatusLate:
 		return 1
-	case "PENDING":
+	case models.StatusPending:
 		return 3
 	default:
 		return 2