fix: six small fixes — rate limiter leak, DST SLA, probe sort, TUI cleanup

1. Rate limiter cleanup goroutine now stoppable via Stop() channel
   instead of looping forever. Prevents goroutine leak in tests.

2. Dead WindowSizeMsg branch in handleFormMsg removed — top-level
   Update handles resize before forms see it.

3. Probe results sorted by node ID — map iteration no longer
   reorders rows every render.

4. fmtAlertConfig takes models.AlertConfig directly instead of an
   anonymous struct the caller builds inline.

5. Backspace no longer aliases delete — d is the documented key.
   Prevents accidental delete-confirm on habitual backspace.

6. SLA daily buckets use time.Date day arithmetic instead of
   Add(-i*24h) — lands on midnight correctly across DST transitions.

internal/tui/tab_alerts.go

@@ -75,10 +75,7 @@ func fmtAlertType(t string) string {
 	}
 }
 
-func (m Model) fmtAlertConfig(alert struct {
-	Type     string
-	Settings map[string]string
-}) string {
+func (m Model) fmtAlertConfig(alert models.AlertConfig) string {
 	switch alert.Type {
 	case "email":
 		host := alert.Settings["host"]
@@ -201,10 +198,7 @@ func (m Model) viewAlertsTab() string {
 					m.fmtAlertHealth(h),
 					m.zones.Mark(fmt.Sprintf("alert-%d", i), limitStr(a.Name, nameW-2)),
 					fmtAlertType(a.Type),
-					limitStr(m.fmtAlertConfig(struct {
-						Type     string
-						Settings map[string]string
-					}{a.Type, a.Settings}), cfgW-2),
+					limitStr(m.fmtAlertConfig(a), cfgW-2),
 					m.fmtAlertLastSent(h),
 				})
 			}