Stats heatmap: empty hours same height; cname_chain only for long chains

The "Busiest times" grid's hours without data used the class "empty",
which picked up the padding of empty messages. One CNAME to another no
longer warns; three or more get an info note showing the chain.

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

CHANGELOG.md

@@ -42,6 +42,11 @@ All notable changes to Jarl are documented here. The format follows
 
 ### Changed
 
+- `cname_chain` no longer warns about one CNAME pointing at another (a
+  service name at a host alias is common, and named answers the chain at
+  once). A chain of three or more CNAMEs gets an info note that shows
+  it.
+
 - A certificate's private key is only exported over HTTPS, the local
   socket, or to the server itself (plain HTTP from another host is
   refused with `https_required`).
@@ -70,6 +75,10 @@ All notable changes to Jarl are documented here. The format follows
 
 ### Fixed
 
+- The statistics' "Busiest times" grid drew hours without data taller
+  than the others (they picked up the page's padding for empty
+  messages).
+
 - A token for one view of a linked zone (such as an ACME token with
   `views=external`) couldn't add records: a new record is shared, and
   that reached the other view. A new name and type is now local to the