Disable zones instead of deleting them

A disabled zone stays in Jarl with its records, options and history,
still editable, but named doesn't load it; enabling it serves it as it is
then. Signed zones, and zones another view shares, can't be disabled.
jarl zone disable|enable, the zone page's Disable/Enable button.
Database schema 7.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/jarld/main.go

@@ -167,7 +167,7 @@ func zoneIndex(a *app) stats.ZoneIndex {
 		}
 		out := map[string][]string{}
 		for _, z := range st.Zones {
-			if z.Type != model.ZonePrimary && z.Type != model.ZoneSecondary && z.Type != model.ZoneMirror {
+			if z.Type != model.ZonePrimary && z.Type != model.ZoneSecondary && z.Type != model.ZoneMirror || z.Disabled {
 				continue
 			}
 			v := st.ViewName(z)